YALE   UNIVERSITY 
MRS.  HEPSA  ELY  SILLIMAN  MEMORIAL  LECTURES 


RADIOACTIVE    TRANSFORMATIONS 


SILLIMAN  MEMORIAL  LECTURES 

PUBLISHED  BY  CHARLES  SCRIBNER'S  SONS 

ELECTRICITY   AND    MATTER 

By  PROF.  J.  J.  THOMSON.    Net  $1.25 

THE    INTEGRATIVE    ACTION    OF    THE 
NERVOUS    SYSTEM 

By  PROF.  C.  S.  SHERRINGTON.    Net  $3.50 

RADIOACTIVE   TRANSFORMATIONS 

By  PROF.  E.  RUTHERFORD.    Net  $3.50 


RADIOACTIVE 
TRANSFORMATIONS 


BY 

E.  RUTHERFORD,  D.Sc.,  LL.D.,  F.R.S. 

Mac  dona  Id  Professor  of  Physics,  Me  Gil  I  University, 
Montreal 


WITH    DIAGRAMS 


OF  THE 

UNIVERSITY 

OF 


CHARLES   SCRIBNER'S  SONS 
NEW  YORK  1906 


Copyright,  1906, 
BY  YALE  UNIVERSITY. 


Published  September,  1906 


THE   UNIVERSITY   PRESS,   CAMBRIDGE,   U.S.A. 


THE    SILLIMAN    FOUNDATION 

IN  the  year  1883  a  legacy  of  eighty  thousand  dollars 
was  left  to  the  president  and  Fellows  of  Yale  College  in 
the  city  of  New  Haven,  to  be  held  in  trust,  as  a  gift 
from  her  children,  in  memory  of  their  beloved  and  hon- 
ored mother  Mrs.  Hepsa  Ely  Silliman. 

On  this  foundation  Yale  College  was  requested  and 
directed  to  establish  an  annual  course  of  lectures  de- 
signed to  illustrate  the  presence  and  providence,  the 
wisdom  and  goodness  of  God,  as  manifested  in  the  natural 
and  moral  world.  These  were  to  be  designated  as  the 
Mrs.  Hepsa  Ely  Silliman  Memorial  Lectures.  It  was 
the  belief  of  the  testator  that  any  orderly  presentation 
of  the  facts  of  nature  or  .history  contributed  to  the  end 
of  this  foundation  more  effectively  than  any  attempt  to 
emphasize  the  elements  of  doctrine  or  of  creed ;  and  he 
therefore  provided  that  lectures  on  dogmatic  or  polemical 
theology  should  be  excluded  from  the  scope  of  this  foun- 
dation, and  that  the  subjects  should  be  selected  rather 
from  the  domains  of  natural  science  and  history,  giving 
special  prominence  to  astronomy,  chemistry,  geology,  and 
anatomy. 

It  was  further  directed  that  each  annual  course  should 
be  made  the  basis  of  a  volume  to  form  part  of  a  series 
constituting  a  memorial  to  Mrs.  Silliman.  The  memo- 
rial fund  came  into  the  possession  of  the  Corporation 
of  Yale  University  in  the  year  1902;  and  the  present 
volume  constitutes  the  third  of  the  series  of  memorial 
lectures. 


i 


PREFACE 

THE  present  work  contains  the  subject  matter  of  eleven 
lectures  delivered  under  the  Silliman  Foundation  at 
Yale  University,  March,  1905. 

I  chose  as  the  subject  of  my  lectures  the  most  recent 
and  at  the  same  time  the  most  interesting  development 
of  Radioactivity,  namely  the  transformations  which  are 
continuously  taking  place  in  radioactive  matter.  While 
dealing  fully  with  this  aspect  of  the  subject,  it  was  neces- 
sary for  clearness  to  give  some  account  of  radioactive 
phenomena  in  general,  although  with  much  less  com- 
pleteness than  in  my  previous  book  on  Radioactivity. 

In  arranging  the  chapters  of  the  present  volume,  the 
order  in  which  the  subject  was  dealt  with  in  the  lectures 
has  been  closely  followed,  but  as  our  knowledge  of  the 
subject  is  increasing  so  rapidly,  I  have  thought  it  desirable 
to  incorporate  the  results  of  the  many  important  inves- 
tigations which  have  been  made  since  the  lectures  were 
delivered.  This  is  .especially  the  case  in  the  chapter 
dealing  with  the  a  rays,  to  which  much  attention  has 
been  devoted  in  the  past  year  on  account  of  the  im- 
portant part  they  play  in  radioactive  transformations. 

I  am  much  indebted  to  my  colleagues  Professor  Hark- 
ness  and  Professor  Brown  for  the  great  care  and  trouble 
they  have  taken  in  the  correction  of  the  proofs  and  for 
many  useful  suggestions. 

E.  RUTHERFORD. 

McGiLL  UNIVERSITY, 

MONTREAL,  June  4,  1906. 


CONTENTS 

CHAPTER  PAGE 

I.    HISTORICAL  INTRODUCTION i 

II.    RADIOACTIVE  CHANGES  IN  THORIUM 37 

III.  THE  RADIUM  EMANATION 70 

IV.  TRANSFORMATION  OF  THE  ACTIVE  DEPOSIT  OF  RADIUM   .  95 
V.    ACTIVE  DEPOSIT  OF  RADIUM  OF  SLOW  TRANSFORMATION  .  122 

VI.    ORIGIN  AND  LIFE  OF  RADIUM 148 

VII.    TRANSFORMATION  PRODUCTS  OF  URANIUM  AND  ACTINIUM, 

AND  THE  CONNECTION  BETWEEN  THE  RADIOELEMENTS  162 
VIII.    THE   PRODUCTION   OF  HELIUM   FROM   RADIUM   AND   THE 

TRANSFORMATION  OF  MATTER 179 

IX.    RADIOACTIVITY  OF  THE  EARTH  AND  ATMOSPHERE   .     .     .  196 

X.    PROPERTIES  OF  THE  «  RAYS .  219 

XL    PHYSICAL  VIEW  OF  RADIOACTIVE  PROCESSES 256 

INDEX     ......                                      277 


RADIOACTIVE 
TRANSFORMATIONS 

CHAPTER  I 
HISTORICAL  INTRODUCTION 

THE  last  decade  has  been  a  very  fruitful  period  in  physical 
science,  and  discoveries  of  the  most  striking  interest  and 
importance  have  followed  one  another  in  rapid  succession. 
Although  the  additions  to  our  knowledge  have  come  from 
investigations  in  very  different  fields,  yet  a  close  examination 
shows  that  they  are  all  intimately  related,  and  each  discovery 
has  supplied  the  necessary  stimulus  and  suggestion  to  serve  as 
a  starting  point  for  the  next  advance. 

The  march  of  discovery  has  been  so  rapid  that  it  has  been 
difficult  even  for  those  directly  engaged  in  the  investigations  to 
grasp  at  once  the  full  significance  of  the  facts  that  have  been 
brought  to  light.  Especially  has  this  been  the  case  in  the  field 
of  radioactivity,  where  the  phenomena  observed  have  been  so 
complicated  and  the  laws  controlling  them  so  unusual  that  it 
has  been  necessary  to  introduce  conceptions  of  a  novel  character 
for  their  explanation. 

The  starting  point  of  this  epoch  in  physical  science  was  the 
discovery  by  Rontgen  of  the  X-rays  in  1895  and  the  experiments 
of  Lenard  on  the  cathode  rays.  The  extraordinary  properties 
of  the  X-rays  at  once  focussed  the  attention  of  the  scientific 
world,  and  led  to  a  series  of  investigations  whose  object  was 
not  only  to  examine  the  properties  of  the  rays  themselves,  but 
to  disclose  their  real  nature  and  origin. 

The  latter  problem  led  to  a  much  closer  investigation  of  the 
nature  of  the  cathode  rays  produced  in  a  vacuum  tube,  for 
these  rays  were  seen  to  be  in  some  way  intimately  connected 
with  the  emission  of  X-rays.  J.  J.  Thomson  in  1897  finally 

l 


2  RADIOACTIVE   TRANSFORMATIONS 

succeeded  in  proving  definitely  that  the  cathode  rays  consisted 
of  a  stream  of  particles  moving  with  great  velocities  and  carry- 
ing negative  charges  of  electricity.  These  particles  had  an 
apparent  mass  only  about  T^<j^  that  of  the  hydrogen  atom,  and 
were  therefore  the  smallest  bodies  known  to  science.  These 
"corpuscles,"  or  " electrons, "  as  they  have  been  termed,  are 
apparently  a  constituent  of  all  matter,  and  are  believed  to  be 
the  ultimate  parts  of  which  the  atom  is  composed. 

This  electronic  hypothesis  has  been  extremely  fertile,  and  has 
already  greatly  changed  —  or  rather  extended  —  previous  concep- 
tions of  the  constitution  of  matter.  It  has  opened  up  wide 
fields  of  investigation  in  many  departments  of  physical  science, 
and  has  provided  science  with  a  microscope,  so  to  speak,  with 
which  to  examine  the  structure  of  the  atom  of  the  chemist. 
J.  J.  Thomson  has  examined  mathematically  the  stability  of 
atoms  composed  of  a  number  of  whirling  electrons,  and  has 
shown  that  these  model  atoms  imitate  in  a  remarkable  way  some 
of  the  more  fundamental  properties  of  the  chemical  atom. 

The  proof  of  the  corpuscular  character  of  the  cathode  rays  at 
once  indicated  the  probable  explanation  of  the  origin  and  nature 
of  the  X-rays.  Stokes,  J.  J.  Thomson,  and  Weichert  independ- 
ently suggested  that  the  cathode  rays  were  the  parents  of  the 
X-rays.  The  sudden  stoppage  of  the  electrons  in  the  cathode 
stream  causes  an  intense  electromagnetic  disturbance  which 
travels  out  from  the  point  of  impact  with  the  velocity  of  light. 
On  this  point  of  view,  the  X-rays  consist  of  a  number  of  dis- 
connected pulses,  following  one  another  in  rapid  succession 
but  without  any  definite  order.  They  are  akin  in  some  respects 
to  very  short  waves  of  ultra-violet  light,  but  differ  from  them  in 
the  lack  of  periodicity  of  the  pulses.  The  penetrating  power  of 
the  rays,  and  the  absence  of  any  direct  reflection,  refraction, 
or  polarization,  were  consequences  of  this  theory  if  the  breadth 
of  the  pulse  was  short  compared  with  the  diameter  of  the  atom. 

An  admirable  and  simple  account  of  the  nature  and  properties 
of  such  pulses  has  been  given  by  J.  J.  Thomson 3  in  the  Silli- 
man  lectures  of  1903. 

1  J.  J.  Thomson:  Electricity  and  Matter  (Scribner,  New  York,  1904). 


HISTORICAL  INTRODUCTION  3 

In  the  meantime,  another  remarkable  property  of  the  X-rays 
had  been  closely  examined.  The  passage  of  the  X-rays  through 
a  gas  imparts  to  it  a  new  power  of  rapidly  discharging  an  electri- 
fied body.  This  was  satisfactorily  explained  on  the  hypothesis 
that  the  rays  produced  a  number  of  positively  and  negatively 
charged  carriers  or  ions  in  the  electrically  neutral  gas.1  The 
development  of  this  subject  proceeded  along  two  distinct  lines, 
one  electrical  and  the  other  optical.  C.  T.  R.  Wilson 2  found 
that  under  certain  conditions  the  ions  produced  in  the  gas  by 
X-rays  become  nuclei  for  the  condensation  of  water  upon  them. 
Each  ion  thus  becomes  the  centre  of  a  visible  charged  globule 
of  water  which  moves  in  an  electric  field.  Experiments  of  this 
character  verified  in  a  remarkable  way  the  fundamental  correct- 
ness of  the  ionization  theory,  and  clearly  brought  out  the  dis- 
continuous or  atomic  structure  of  the  carriers  of  the  electric 
charges. 

As  a  result  of  researches  on  diffusion  of  the  ions  in  erases, 

o 

Townsend  3  deduced  the  important  fact  that  the  charge  carried 
by  a  gaseous  ion  was  the  same  in  all  cases,  and  equal  to  the 
charge  carried  by  the  hydrogen  atom  in  the  electrolysis  of 
water.  By  a  combination  of  the  electrical  and  optical  methods, 
J.  J.  Thomson  4  found  the  actual  value  of  the  charge  carried  by 
an  ion. 

The  determination  of  this  important  physical  unit  at  once 
allows  us  to  count  the  number  of  ions  present  in  any  volume  of 
air  acted  upon  by  an  ionizing  agent.  In  addition  to  this,  it  pro- 
vides the  most  accurate  deduction  yet  made  of  the  number  of 
molecules  present  in  unit  volume  of  any  gas  at  standard  pres- 
sure and  temperature.  This  number,  which  is  based  purely  on 
experimental  data,  will  be  seen  to  be  of  the  greatest  value  in 
calculating  the  magnitudes  of  various  quantities  in  the  subject 
of  radioactivity. 

The  ionization  theory  of  gases  was  successfully  applied  to 

1  J.  J.  Thomson  and  E.  Rutherford :  Phil.  Mag.,  Nov.,  1896. 

2  C.  T.  R.  Wilson:  Phil.  Trans., p.  265,  1897  ;  p.  403,  1899;  p.  289,  1900. 

3  Townsend:  Phil.  Trans.  A,  p.  129,  ^SW.  ?^00  ItAtffy) 

4  J.  J.  Thomson:  Phil.  Mag.,  Dec.,  1898;  March,  1903. 


4  RADIOACTIVE  TRANSFORMATIONS 

account  for  the  conductivity  of  flames  and  heated  vapors  and  to 
unravel  the  complicated  phenomena  observed  in  the  discharge 
of  electricity  through  a  vacuum  tube.  This  fascinating  and  far- 
reaching  field  of  physical  inquiry  owes  its  inception  and  much 
of  its  development  to  Prof.  J.  J.  Thomson  and  his  students 
at  the  Cavendish  Laboratory,  Cambridge. 

On  the  theoretical  side  the  possibilities  of  an  ionic  or  elec- 
tronic theory  of  matter  had  been  recognized  long  before  the 
experimental  evidence  was  forthcoming.  The  most  notable  ex- 
ponents of  this  school  were  Lorentz  and  Larmor,  who  developed 
their  theories  to  account,  among  other  things,  for  the  mechan- 
ism of  radiation.  The  discovery  by  Zeeman  of  the  action  of  a 
magnetic  field  in  displacing  the  spectral  lines  afforded  a  strong 
confirmation  of  the  general  theory,  for  the  experimental  results 
observed  were  in  large  part  predicted  by  the  theory  of  Lorentz. 
In  addition  it  was  deduced  that  the  ion,  whose  movements  gave 
rise  to  the  radiation,  had  a  mass  of  about  the  same  small  value 
as  the  corpuscle  of  J.  J.  Thomson  observed  in  a  vacuum  tube. 
Results  of  this  character  at  once  extended  the  range  of  ionic 
theories  to  matter  in  general,  and  though  much  still  remains 
to  be  done,  the  electronic  theory  has  already  proved  of  great 
value  in  elucidating  the  connection  between  some  of  the  most 
recondite  physical  phenomena. 

The  movement  set  on  foot  by  the  discovery  of  Rontgen  had 
even  more  important  consequences  in  another  very  unexpected 
direction.  Immediately  after  the  discovery  of  the  X-rays,  it 
was  thought  that  the  emission  of  these  rays  was  in  some  way 
connected  with  the  phosphorescence  set  up  by  the  cathode  rays 
on  the  walls  of  a  vacuum  tube. 

It  occurred  to  several  scientists  that  natural  bodies  which 
phosphoresced  under  the  influence  of  light  might  possess 
the  property  of  emitting  a  penetrating  type  of  radiation 
similar  to  X-rays.  We  now  know  that  this  speculation 
had  no  secure  basis  in  fact,  but  it  provided  the  stimulus 
for  the  investigation  of  the  properties  of  substances  in  this 
special  direction  and  soon  led  to  a  discovery  of  far-reaching 
importance. 


HISTORICAL  INTRODUCTION  5 

M.  Henri  Becquerel,1  a  most  distinguished  French  physicist, 
in  pursuance  of  this  idea  exposed  amongst  other  substances  a 
phosphorescent  uranium  compound  —  the  double  sulphate  of 
uranium  and  potassium  —  to  a  photographic  plate  enveloped  in 
black  paper.  A  darkening  of  the  plate  was  observed,  showing 
that  this  substance  emitted  rays  capable  of  passing  through 
matter  opaque  to  ordinary  light.  It  was  soon  found,  however, 
that  this  property  was  not  in  any  way  the  result  of  phosphores- 
cence, for  it  was  exhibited  by  all  the  compounds  of  uranium 
and  the  metal  itself,  even  if  these  had  been  kept  for  a  long  time 
in  a  dark  room. 

The  radiations  emitted  from  uranium  were  found  to  be  similar 
to  X-rays  in  their  penetrating  powers.  It  was  at  first  thought 
that  they  differed  from  X-rays  in  showing  some  evidence  of 
reflection,  refraction,  and  polarization,  but  this  was  found  later 
to  be  incorrect. 

Becquerel  observed  that  the  uranium  rays,  in  addition  to  their 
photographic  action,  possessed,  like  X-rays,  the  important  prop- 
erty of  discharging  an  electrified  body.  This  was  later  examined 
in  detail  by  the  writer,2  who  found  that  this  discharging  action 
could  be  explained  on  the  assumption  that  the  gas  was  ionized 
by  the  passage  of  the  radiations  through  it.  The  ions  were 
found  to  be  identical  with  those  produced  by  X-rays,  and  the 
ionization  theory  could  consequently  be  directly  applied  to  ex- 
plain the  various  discharge  phenomena  produced  by  the  rays 
from  uranium.  At  the  same  time,  it  was  clearly  brought  out 
that  the  rays  from  uranium  consisted  of  two  distinct  kinds, 
called  the  a  and  &  rays.  The  former  were  very  easily  absorbed 
in  air  and  in  thin  sheets  of  foil,  while  the  latter  were  of  a  far 
more  penetrating  type. 

The  intensity  of  the  radiations  emitted  by  uranium,  whether 
examined  by  the  photographic  or  electrical  method,  remains  con- 
stant, or  at  any  rate  changes  extremely  slowly,  for  no  appreciable 
alteration  has  been  observed  over  a  period  of  several  years. 
The  photographic  and  electrical  effects  exhibited  by  uranium 

1  Becquerel:  Comptes  rendus,  cxxii,  pp.  420,  501,  559,  689,  762,  1086  (1896). 

2  Rutherford:  Phil.  Mag.,  Jan.,  1899. 


6  RADIOACTIVE   TRANSFORMATIONS 

are  very  feeble  compared  with  those  produced  by  the  X-rays 
from  the  ordinary  focus  tube.  An  exposure  to  uranium  salts 
of  at  least  one  day  is  required  to  produce  any  marked  action  on 
the  plate. 

The  term  "radioactivity"  is  now  generally  understood  to 
signify  the  property  shown  by  the  class  of  substances,  of  which 
uranium,  thorium,  and  radium  are  the  best-known  examples, 
of  spontaneously  emitting  special  types  of  radiations  capable  of 
acting  on  a  photographic  plate  and  of  discharging  an  electrified 
body.  The  term  "  activity  "  is  used  to  denote  the  intensity  of 
the  electrical  or  other  effect  of  the  rays  from  a  substance  com- 
pared with  that  shown  by  some  standard  substance.  Uranium 
is  usually  chosen  as  this  standard  substance,  in  consequence  of 
the  constancy  of  its  radiations,  and  the  activity  exhibited  by 
other  bodies  is  usually  expressed  in  terms  of  the  ratio  of  the  elec- 
trical effect  produced  by  the  active  matter  under  consideration 
compared  with  that  of  an  equal  weight  either  of  uranium  metal 
itself  or  of  uranium  oxide  spread  over  an  equal  radiating  area. 
For  example,  when  the  activity  of  radium  is  said  to  be  about 
two  millions,  it  is  meant  that  the  electrical  effect  due  to  it  is 
two  million  times  as  great  as  the  corresponding  effect  produced 
by  an  equal  weight  of  uranium  spread  over  an  equal  area. 

Although  the  property  possessed  by  uranium  of  spontaneously 
emitting  energy  in  special  forms  without  any  apparent  change 
in  the  matter  itself  could  riot  fail  to  be  regarded  as  a  most  re- 
markable phenomenon,  yet  the  rate  of  emission  of  energy,  judged 
by  ordinary  standards,  is  so  feeble  that  it  did  not  attract  that 
active  scientific  attention  which  was  afterwards  excited  by  the 
discovery  of  radium ;  for  this  substance  exhibited  the  properties 
of  uranium  to  such  a  remarkable  degree  that  it  impressed  both 
the  lay  and- the  scientific  mind. 

Shortly  after  the  discovery  of  Becquerel,  Mme.  Curie 1  made  a  / 
systematic  examination  of  different  substances  for  radioactivity, 
and  found  that  the  element  thorium  possessed  a  similar  property 
to  uranium  and  almost  to  the  same  degree.     This  fact  was  also 

1  Mme.  Curie:  Comptes  rendus,  cxxvi,  p.  1101  (1898). 


HISTORICAL   INTRODUCTION  7 

Independently  observed  by  Schmidt.1  An  examination  was 
then  made  of  the  natural  minerals  which  contain  thorium  and 
uranium,  and  here  an  unexpected  result  was  observed.  Some 
of  these  minerals  were  found  to  be  several  times  more  radioac- 
tive than  pure  uranium  or  thorium,  and  in  all  cases  the  uranium 
minerals  showed  four  to  five  times  the  activity  to  be  expected 
from  the  percentage  of  uranium  present.  Mme.  Curie  found 
that  the  radioactivity  of  uranium  was  an  atomic  property,  i.  e., 
the  activity  observed  depended  only  on  the  amount  of  the  ele- 
ment uranium  present,  and  was  not  affected  by  its  combination 
with  other  substances.  This  being  so,  the  large  activity  of  the 
uranium  minerals  could  only  be  accounted  for  by  supposing  that 
another  unknown  substance  was  present,  which  was  far  more 
active  than  uranium  itself. 

Relying  on  this  hypothesis,  Mme.  Curie  boldly  proceeded  to 
see  if  it  were  possible  to  separate  chemically  this  unknown  active 
substance  from  uranium  minerals.  By  the  courtesy  of  the  Aus- 
trian Government,  she  was  presented  with  a  ton  of  uranium 
residues  from  the  State  Manufactory  at  Joachimsthal,  Bohemia. 
In  this  locality  there  are  extensive  deposits  of  uraninite,  com- 
monly called  pitchblende,  which  are  mined  for  the  uranium  they 
contain.  This  pitchblende  consists  mainly  of  uranium,  but  also 
contains  small  quantities  of  a  number  of  rare  elements. 

As  a  guide  to  the  separation  of  the  active  substance,  Mme. 
Curie  employed  a  suitable  electroscope  to  measure  the  ioniza- 
tion  produced  by  the  active  body.  After  any  chemical  separation, 
the  activities  of  the  precipitate  and  of  the  filtrate  evaporated  to 
dryness  were  separately  examined,  and  in  this  way  it  was  possi- 
ble to  ascertain  whether  the  active  substance  had  been  mainly 
precipitated  or  left  behind  in  the  filtrate. 

The  electric  method  was  thus  used  as  a  rapid  means  of  qualita- 
tive and  quantitative  analysis.  Proceeding  in  this  way,  Mme. 
Curie  found  that  not  one  but  two  very  active  substances  were 
present  in  the  uranium  residues.  The  former  of  these,  which 
was  separated  with  bismuth,  was  called  polonium,2  in  honor  of 

1  Schmidt:  Annal.  d.  Phys.,  Ixv,  p.  141  (1898). 

2  Mme.  Curie:  Comptes  reudus,  oxxvii,  p.  175  (1898). 


8  RADIOACTIVE   TRANSFORMATIONS 

the  country  of  her  birth,  and  the  latter,  which  was  separated  with 
barium,  was  called  radium.1  This  latter  name  was  a  happy  in- 
spiration, for  the  activity  of  this  substance  in  a  pure  state  is  at 
least  two  million  times  that  of  uranium.  Mme.  Curie  then 
proceeded  with  the  laborious  work  of  separating  the  radium 
from  the  barium,  and  was  finally  successful  in  obtaining  a  small 
quantity  of  probably  pure  radium  chloride.  The  atomic  weight 
was  found  to  be  225.  The  spectrum,  first  examined  by  De- 
margay,  was  found  to  consist  of  a  number  of  bright  lines,  and 
was  analogous  in  many  respects  to  the  spectra  of  the  alkaline 
earths. 

In  chemical  properties  radium  is  closely  allied  to  barium, 
but  can  be  completely  separated  from  it  by  taking  account  of 
the  differences  in  the  solubility  of  the  chlorides  and  bromides. 
On  account  of  the  small  quantities  of  radium  compounds  avail- 
able and  of  their  great  cost,  no  one  has  yet  endeavored  to  obtain 
radium  in  a  metallic  state.  Marckwald,2  however,  electrolyzed 
a  radium  solution  with  a  mercury  cathode,  and  concluded  that 
the  metal  forms  an  amalgam  with  the  mercury  in  the  same  way 
as  barium.  The  small  trace  of  the  metal  so  obtained  exhibited 
the  characteristic  radiating  properties  of  the  radium  compounds. 

There  cannot  be  the  slightest  doubt  that  when  radium  is 
obtained  in  the  metallic  state  it  will  be  radioactive;  for  the 
property  of  radioactivity  is  atomic  and  not  molecular.  In 
addition,  uranium  and  thorium  as  metals  exhibit  the  activity 
to  be  expected  from  an  examination  of  the  activities  of  their 
compounds. 

Radium  exists  in  very  small  quantity  in  radioactive  minerals. 
It  will  be  seen  later  that  the  amount  of  radium  in  different 
minerals  is  always  proportional  to  their  content  of  uranium. 
The  amount  of  radium  per  ton  of  uranium  is  about  .35  gram, 
or  less  than  one  part  in  a  million  of  the  mineral.  From  a 
ton  of  Joachimsthal  uraninite,  which  contains  about  50  per 
cent  of  uranium,  the  theoretical  yield  of  radium  should  be  about 
.17  gram. 

1  M.  and  Mme.  Curie  and  G.  Bemont:  Comptes  rendus,  cxxvii,  p.  12T5  (1898). 

2  Marckwald  :  Ber.  d.  d.  chem.  Ges.,  No.  1,  p.  88,  1904. 


HISTORICAL  INTRODUCTION  9 

In  order  to  separate  the  radium  from  the  barium  mixed  with 
it,  Mme.  Curie  employed  the  method  of  fractional  crystalliza- 
tion of  the  chlorides.  Giesel 1  found  that  by  use  of  the  bromide 
instead  of  the  chloride  the  separation  of  radium  from  barium 
was  much  facilitated.  He  states  that  six  crystallizations  suffice 
almost  completely  to  remove  the  radium  from  the  barium. 

The  discovery  of  radium  gave  a  great  impetus  to  the  chemical 
examination  of  radioactive  minerals  in  order  to  see  if  other  radio- 
•active  substances  were  present.  Debierne 2  succeeded  in  ob- 
taining a  new  radioactive  body  called  "actinium."  Giesel1 
independently  observed  the  presence  of  a  new  radiating  body 
which  he  called  the  "emanating  substance,"  and  later  "eman- 
ium,"  on  account  of  the  rapid  emission  from  it  of  a  short-lived 
radioactive  emanation  or  gas.  Recent  work  has  shown  that  the 
substances  separated  by  Debierne  and  Giesel  are  identical  in 
radioactive  properties  and  must  contain  the  same  element. 
Hofmann  and  Strauss  3  separated  an  active  substance  which  was 
precipitated  with  lead  and  called  by  them  "radiolead,"  while 
Marckwald 4  later  obtained  from  pitchblende  residues  some  ex- 
tremely active  matter  which  he  named  "  radiotellurium, ' '  since 
it  was  initially  separated  with  tellurium  as  an  impurity. 

None  of  these  active  bodies  except  radium  have  yet  been  ob- 
tained in  a  pure  state.  We  shall  see  later  that  the  active  ele- 
ment present  in  the  radiotellurium  of  Marckwald  is  almost 
certainly  identical  with  that  in  the  polonium  of  Mme.  Curie; 
it  will  also  be  shown  that  the  active  elements  present  in  radio- 
lead  and  radiotellurium  are  in  reality  produced  from  the  radium 
present  in  pitchblende,  or,  in  other  words,  both  are  products 
of  the  transformation  of  the  radium  atom. 

The  possibility  of  using  very  active  preparations  of  radium 
as  a  source  of  radiation  led  to  a  close  examination  of  the  nature 
of  the  rays  emitted  so  freely  from  this  substance.  Giesel 6  ob- 
served in  1899  that  the  more  penetrating  rays,  known  as  /3  rays, 

1  Giesel:  Ber.  d.  d.  chem.  Ges.,  p.  3608,  1902  ;  p.  342,  1903. 

2  Debierne:  Comptes  rendus,  cxxix,  p.  593  (1899) ;  cxxx,  p.  206  (1900). 
8  Hofmann  and  Strauss:  Ber.  d.  d.  chem.  Ges.,  p.  3035,  1901. 

*  Marckwald  :  Ibid.,  p.  2662,  1903. 

5  Giesel:  Annal.  d.  Phys.,  Ixix,  p.  834  (1899). 


10  RADIOACTIVE   TRANSFORMATIONS 

were  deflected  in  a  magnetic  field  in  the  same  direction  as  the 
cathode  rays,  indicating  that  they  consisted  of  negatively  charged 
particles  projected  with  great  speed  from  the  active  substance. 

This  was  substantiated  by  experiments  of  Becquerel,1  who 
examined  the  deviation  of  a  pencil  of  rays  both  in  a  magnetic  and 
electric  field.  His  results  showed  that  the  /3  particles  had  the 
same  small  mass  as  the  particles  of  the  cathode  stream,  whose 
corpuscular  nature  had  previously  been  demonstrated  by  J.  J. 
Thomson.  The  ft  particle  was  in  fact  identical  with  the  elec- 
tron set  free  by  the  electric  discharge  in  a  vacuum  tube. 

The  /3  particles  were  projected  from  radium  at  different 
speeds,  but  their  average  velocity  was  much  greater  than  that 
impressed  on  the  electron  in  a  vacuum  tube,  and  in  many  cases 
approached  closely  to  the  velocity  of  light.  This  property  of 
radium  of  emitting  a  stream  of  fi  particles  at  different  speeds 
was  later  utilized  by  Kaufmann 2  to  determine  the  variation  of 
the  mass  of  the  ft  particle  with  velocity.  J.  J.  Thomson  had 
shown  in  1887  that  a  charged  body  in  motion  possessed  electri- 
cal mass  in  virtue  of  its  motion.  The  theory  of  this  action  was 
later  developed  by  Heaviside,  Searle,  Abraham,  and  others. 

The  moving  charge  acts  as  an  electric  current,  and  a  magnetic 
field  is  generated  round  the  body  and  moves  with  it.  Magnetic 
•energy  is  stored  in  the  medium  surrounding  the  charged  body, 
which  consequently  behaves  as  if  it  had  a  greater  apparent  mass 
than  when  uncharged.  This  additional  electric  mass,  accord- 
ing to  the  theory,  should  be  constant  for  small  speeds  but  should 
increase  rapidly  as  the  velocity  of  light  is  approached. 

Kaufmann  found  from  his  experiments  that  the  apparent  mass 
of  the  electron  did  increase  with  speed,  and  that  the  increase  was 
rapid  as  the  velocity  of  light  was  approached.  By  comparing 
the  theory  with  experiment,  he  concluded  that  the  apparent  mass 
of  the  ft  particle  was  entirely  electrical  in  origin,  and  that  there 
was  no  necessity  to  assume  the  presence  of  a  material  nucleus 
on  which  the  charge  was  distributed. 

This  was  a  very  important  result,  for  it  indirectly  offered  a 

1  Becquerel :  Comptes  rend  us,  cxxx,  p.  809  (1900). 

2  Kaufmann :  Physik.  Zeit,  iv,  No.  1  b,  p.  54  (1902). 


HISTORICAL   INTRODUCTION  11 

possible  explanation  of  the  origin  of  mass,  which  has  always 
been  such  an  enigma  to  science.  If  a  charge  of  electricity  in 
motion  exactly  simulates  the  properties  of  mechanical  mass,  it 
is  possible  that  the  mass  of  matter  in  general  may  be  electrical 
in  origin,  and  may  result  from  the  movement  of  the  electrons 
constituting  the  molecules  of  matter.  Such  a  point  of  view, 
while  most  suggestive  and  important,  cannot  at  present  be  con- 
sidered more  than  a  justifiable  speculation. 

Villard 1  in  1900  found  that  radium  emitted  in  addition  to  a 
and  fi  rays,  a  third  type  of  radiation,  now  called  the  7  rays, 
which  are  of  an  extremely  penetrating  character.  These  rays 
are  undeflected  by  a  magnetic  or  electric  field  and  appear  to  be 
a  type  of  penetrating  X-rays,  which  accompany  the  expulsion  of 
the  0  particles  from  radium.  The  presence  of  these  rays  was 
also  observed  later  in  thorium,  uranium,  and  actinium. 

In  the  meantime,  the  importance  of  the  a  rays  began  to  be 
more  clearly  recognized.  These  rays  do  not  possess  much  power 
of  penetrating  matter,  for  they  are  stopped  in  their  passage 
through  a  few  centimetres  of  air  and  by  a  few  thicknesses  of 
metal  foil.  On  the  other  hand,  they  produce  far  more  ioniza- 
tion  in  the  gas  than  the  y8  and  7  rays,  and  the  greater  proportion 
of  the  energy  radiated  from  radioactive  bodies  is  in  the  form  of 
these  rays.  They  were  at  first  thought  to  be  non-deflectable  in 
a  magnetic  field,  but  in  1902  the  writer  2  showed  that  they  were 
deflected  to  a  measurable  extent  in  strong  magnetic  and  electric 
fields.  The  direction  of  deflection  was  opposite  to  that  of  the 
/3  particles,  showing  that  they  carry  a  positive  and  not  a  nega- 
tive charge  of  electricity. 

From  measurements  of  the  amount  of  deflection  of  the  rays 
both  in  a  magnetic  and  electric  field,  it  was  found  that  the 
a  particle  from  radium  was  projected  with  a  velocity  about  ^ 
that  of  light  and  had  a  mass  about  twice  that  of  the  hydrogen 
atom.  The  a  rays  from  radium  thus  consist  of  a  stream  of  atoms 
of  matter  projected  with  great  velocity.  We  shall  see  later  that 
there  is  some  reason  to  believe  that  the  a  particle  is  an  atom  of 

1  Villard:  Comptes  rendus,  cxxx,  pp.  1010,  1178  (1900). 

2  Rutherford  :  Phil.  Mag.,  Feb.,  1903;  Physik.  Zeit.,  iv,  p.  235  (1902). 


12  RADIOACTIVE   TRANSFORMATIONS 

helium.  The  main  radiations  frem  radium  are  thus  corpuscular 
in  character,  and  consist  of  streams  of  positively  and  negatively 
charged  particles. 

In  1903,  Sir  William  Crookes,1  and  Elster  and  Geitel,2  inde- 
pendently observed  a  very  interesting  property  of  the  a  rays. 
The  a  rays  from  radium  or  other  strongly  active  substance 
produce  phosphorescence  on  a  screen  of  crystalline  zinc  sul- 
phide (Sidot's  blende).  On  examination  of  the  screen  with  a 
lens,  the  luminosity  is  found  to  be  not  uniform,  but  to  consist 
of  a  number  of  bright  points  of  light,  which  follow  one  another 
in  irregular  but  rapid  succession.  These  "scintillations  "  are  a 
result,  probably  indirect,  of  the  bombardment  of  the  screen 
by  the  massive  a  particles,  but  the  exact  explanation  of  this 
striking  phenomenon  is  still  unsettled. 

In  the  meantime  the  complexity  of  the  processes  occurring  in 
thorium  and  radium  became  more  evident.  The  writer  3  in  1900 
showed  that  thorium,  in  addition  to  the  expulsion  of  a  and  /3 
particles,  continuously  e'mits  a  radioactive  emanation  or  gas. 
Both  radium  and  actinium  also  exhibit  a  similar  property. 
These  emanations  consist  of  gaseous  radioactive  matter,  the 
radiating  power  of  which  rapidly  dies  away.  The  emanations 
of  thorium,  radium,  and  actinium  can  readily  be  distinguished 
from  one  another  by  the  rate  at  which  they  lose  their  activity. 
The  emanations  of  both  actinium  and  thorium  are  very  short 
lived,  the  former  losing  half  of  its  activity  in  3.9  seconds  and 
the  latter  in  54  seconds.  On  the  other  hand,  the  emanation 
from  radium  is  far  more  persistent,  and  requires  a  lapse  of  about 
four  days  to  reduce  the  activity  to  half  value. 

About  the  same  time  another  remarkable  action  of  radium 
and  thorium  was  disclosed.  M.  and  Mme.  Curie4  found  that 
all  bodies  placed  in  the  neighborhood  of  radium  salts  became 
temporarily  active.  A  similar  property  was  independently  ob- 
served by  the  writer  for  thorium.5  This  property  of  radium  and 

1  Crookes  :  Proc.  Hoy.  Soc.,  Ixxxi,  p.  405  (1903). 

2  Elster  and  Geitel :  Physik.  Zeit.,  No.  15,  p.  437,  1903. 
8  Rutherford  :  Phil.  Mag.,  Jan.  and  Feb.,  1900. 

4  M.  and  Mme.  Curie:  Comptes  rendus,  cxxix,  p.  714  (1899). 
6  Rutherford  :  Phil.  Mag.,  Jan.  and  Feb.,  1900. 


HISTORICAL   INTRODUCTION  13 

thorium  of  "exciting"  or  "inducing"  activity  in  substances 
placed  near  them  is  directly  due  to  the  emanations  from  these 
bodies.  The  emanation  is  an  unstable  substance  and  is  trans- 
formed into  a  non-gaseous  type  of  matter  which  is  deposited  on 
the  surface  of  all  bodies  in  its  neighborhood. 

Another  striking  property  of  radium  was  observed  by  P.  Curie 
and  Laborde  in  1903. l  A  radium  compound  continuously  emits 
heat  at  a  rate  sufficient  to  melt  more  than  its  own  weight  of 
ice  per  hour.  In  consequence  of  this  a  mass  of  radium  always 
keeps  itself  at  a  higher  temperature  than  the  surrounding  air. 
This  rapid  emission  of  heat  by  radium  is  directly  connected 
with  its  radioactive  properties,  and  will  be  shown  later  to  result 
mainly  from  the  bombardment  of  the  radium  by  the  a  particles 
projected  from  its  own  mass. 

From  the  above  brief  review  of  the  more  important  proper- 
ties exhibited  by  the  radioactive  bodies,  it  will  be  seen  that  the 
processes  taking  place  in  a  mass  of  radioactive  matter  are  very 
complicated.  In  a  compound  of  radium,  for  example,  there 
occurs  a  rapid  expulsion  of  a  and  @  particles  accompanied  with 
the  generation  of  7  rays,  a  rapid  emission  of  heat,  the  continuous 
production  of  an  emanation  or  gas,  and  the  formation  of  an  active 
deposit  which  gives  rise  to  "  excited  "  activity. 

A  great  advance  in  the  clear  understanding  of  the  connection 
between  these  various  processes  resulted  from  the  discovery  by 
Rutherford  and  Soddy 2  that  a  very  active  substance,  called 
thorium  X,  could  be  separated  from  thorium  by  a  simple  chemi- 
cal operation.  This  thorium  X  was  found  to  lose  its  activity 
in  time,  while  the  thorium  freed  from  ThX  spontaneously  pro- 
duced a  new  supply  of  ThX.  In  a  mass  of  thorium  in  radio- 
active equilibrium,  the  two  processes  of  growth  and  change  of 
ThX  proceed  simultaneously,  and  the  amount  of  ThX  present 
reaches  a  constant  value  when  its  rate  of  production  from  the 
thorium  balances  its  own  rate  of  change.  It  was  found  that 
the  thorium  "  emanation  ' '  was  directly  produced  by  ThX,  while 

1  P.  Curie  and  Laborde :  Comptes  rendus,  cxxxvi,  p.  673  (1903). 

2  Rutherford  and  Soddy :  Phil.  Mag.,  Sept.  and  Nov.,  1902 ;  Trans.  Chem.  Soc., 
Ixxxi,  pp.  321  and  837  (1902). 


14  RADIOACTIVE    TRANSFORMATIONS 

the  emanation  in  turn  gave  rise  to  the  active  deposit  which 
causes  the  phenomenon  of  excited  activity. 

Now  it  has  been  pointed  out  that  the  radioactive  property  is 
atomic,  and  consequently  must  result  from  a  process  occurring 
in  the  atom  and  not  in  the  molecule.  In  order  to  explain  the 
results  observed,  Rutherford  and  Soddy  advanced  a  theory, 
known  as  the  " disintegration  theory."  It  is  supposed  that  the 
atoms  of  the  radioactive  bodies  are  unstable,  and  that  a  certain 
fixed  proportion  of  them  become  unstable  every  second  and 
break  up  with  explosive  violence,  accompanied  in  general  by 
the  expulsion  of  an  a  or  /3  particle  or  both  together.  The 
residue  of  the  atom,  in  consequence  of  the  loss  of  an  a  particle, 
is  lighter  than  before,  and  becomes  the  atom  of  a  new  substance 
quite  distinct  in  chemical  and  physical  properties  from  its  parent. 
In  thorium,  for  example,  it  is  supposed  that  the  atom  of  ThX 
consists  of  the  thorium  atom  minus  an  a  particle.  Thorium  X 
is  unstable  and  breaks  up  at  a  definite  rate  with  the  expulsion 
of  another  a  particle.  The  residue  of  the  atom  of  ThX  in  turn 
becomes  the  atom  of  the  emanation,  and  this  in  turn  breaks  up 
through  a  further  succession  of  stages. 

The  theory  was  found  to  account  in  a  satisfactory  way  for 
the  processes  occurring  not  only  in  thorium  but  in  all  the  radio- 
active bodies.  On  this  view,  the  radioactive  substances  are 
undergoing  spontaneous  transformation  with  the  appearance  of  a 
number  of  new  kinds  of  matter  which  are  unstable  and  have 
a  limited  life.  The  radiations  accompany  the  transformations 
and  are  produced  as  a  result  of  an  explosive  disturbance  within 
the  atom. 

The  long  continued  emission  of  energy  from  the  radioactive 
bodies  does  not  on  this  view  present  any  fundamental  difficulty 
and  is  in  accordance  with  the  principle  of  the  conservation  of 
energy.  The  matter  loses  in  atomic  energy  at  each  stage  of  the 
transformation,  and  the  energy  radiated  is  derived  from  the  in- 
ternal energy  resident  in  the  atoms  themselves.  The  atom 
is  supposed  to  consist  of  a  number  of  charged  parts  in  rapid 
oscillatory  or  orbital  motion  and  consequently  contains  a  great 
store  of  energy.  Part  of  this  energy  is  kinetic  and  part  poten- 


HISTORICAL   INTRODUCTION  15 

tial,  resulting  from  the  condensation  of  the  electrical  charges 
within  the  minute  volume  of  the  atom.  This  latent  energy  of 
the  atom  does  not  ordinarily  manifest  itself,  since  the  chemical 
and  physical  forces  at  our  disposal  do  not  allow  us  to  break  up 
the  atom.  Part  of  this  energy  is,  however,  released  in  radio- 
active changes  where  the  atom  itself  suffers  disruption  with  the 
expulsion  of  one  of  its  charged  parts  with  great  velocity. 

This  theory  has  proved  of  the  greatest  service  in  correlating 
the  various  phenomena  shown  by  the  active  substances.  In 
many  cases,  it  offers  a  quantitative  as  well  as  a  qualitative 
explanation  of  the  experimental  facts,  and  has  proved  of  great 
value  in  suggesting  new  lines  of  attack. 

In  addition  to  its  aid  in  tracing  the  succession  of  transforma- 
tions which  occur  in  the  radioelements,  this  theory  has  been 
instrumental  in  showing  that  radium  is  produced  from  uranium 
and  that  the  active  constituents  in  radiolead  and  radiotellurium 
result  from  the  transformation  of  radium. 

The  application  of  this  theory  to  the  unravelling  of  the  com- 
plicated series  of  transformations  in  radium,  thorium,  and  ac- 
tinium will  form  the  main  subject  matter  of  this  treatise. 

The  disintegration  theory  received  a  strong  measure  of  sup- 
port from  the  remarkable  observation  of  Ramsay  and  Soddy l 
that  the  rare  gas  helium  was  produced  from  the  radium  emana- 
tion. This  in  itself  supplied  unequivocal  evidence  that  there 
was  an  actual  transformation  of  matter  taking  place  in  radium, 
one  of  the  products  of  which  was  the  inactive  gas  helium. 

It  will  be  seen  later  that  the  weight  of  evidence  points  to  the 
conclusion  that  the  a  particle  from  radium  is  an  atom  of  helium. 
On  this  view  helium  arises  during  the  transformation  of  each 
product  which  emits  a  rays.  Such  a  conclusion,  apart  from 
other  evidence,  is  also  supported  by  the  recent  observation  of 
Debierne  that  helium  is  produced  from  actinium  as  well  as 
from  radium. 

In  the  above  review  we  have  traced  the  main  line  of  advance 
of  knowledge  in  the  field  of  radioactivity,  but  there  has  been  a 
rapid  and  important  advance  in  another  direction. 

1  Ramsay  and  Soddy  :  Proc.  Koy.  Soc.,  Ixxii,  p.  204  (1903) ;  Ixxiii,  p.  341  (1904). 


16  RADIOACTIVE   TRANSFORMATIONS 

•  Elster  and  Geitel1  showed  in  1901  that  radioactive  matter 
existed  in  the  atmosphere.  Later  work  has  shown  that  the 
radioactivity  of  the  atmosphere  is  mainly  due  to  the  presence 
of  the  radium  emanation  which  diffuses  into  the  atmosphere 
from  the  earth.  Elster  and  Geitel  and  others  have  made  an 
extensive  examination  of  the  radioactivity  of  soils,  and  of  well 
and  spring  waters,  and  have  shown  that  there  is  a  very  wide 
diffusion  of  small  quantities  of  radioactive  matter  throughout 
the  crust  of  the  earth  and  in  the  atmosphere.  A  number  of 
investigators  have  entered  this  new  field  of  inquiry,  and  a  large 
amount  of  valuable  data  has  already  been  accumulated. 

While  the  radioactive  elements  exhibit  the  quality  of  radio- 
activity in  a  very  marked  degree,  there  is  an  increasing  body  of 
evidence  that  ordinary  matter  also  possesses  this  property  to  a 
very  minute  extent,  and  that  the  activity  observed  cannot  be 
ascribed  to  the  presence  of  traces  of  the  known  radioelements. 
This  detection  of  the  radioactivity  of  ordinary  matter  has  been 
made  possible  by  the  extraordinary  delicacy  of  the  electrical  test 
for  the  presence  of  radiations  which  are  able  to  ionize  a  gas. 

When  it  is  remembered  that  the  initial  discovery  of  the  radio- 
activity of  uranium  was  made  in  1896,  and  that  the  first  evi- 
dence of  the  presence  of  radium  was  obtained  in  1898,  it  will 
be  seen  how  rapid  has  been  the'  advance  in  our  knowledge  of 
this  complicated  subject.  A  very  large  mass  of  experimental 
facts  has  now  been  accumulated,  and  their  connection  with  one 
another  has  been  made  clear  by  the  adoption  of  a  simple  theory. 
The  rapidity  of  this  advance  has  seldom,  if  ever,  been  equalled 
in  the  history  of  science,  and  it  is  of  interest  to  examine  the 
influences  that  have  made  it  possible. 

It  is  not  due  to  the  number  of  workers  in  the  field,  for  until 
the  last  year  or  two  the  subjept  has  been  represented  by  com- 
paratively few  investigators.  The  main  reason  for  the  rapidity 
of  the  advance  lies  in  the  remarkably  opportune  time  at  which 
the  new  field  was  opened  up,  and  the  influence  upon  it  of  the 
rapid  extension  of  our  knowledge  of  the  passage  of  electricity 
through  gases. 

l  Elster  and  Geitel :  Physik.  Zeit.,  ii,  p.  590  (1901). 


HISTORICAL   INTRODUCTION  17 

In  this  connection  it  is  of  interest  to  note  that  the  discovery 
of  the  radioactive  property  of  uranium  might  accidentally  have 
been  made  a  century  ago,  for  all  that  was  required  was  the  ex- 
posure of  a  uranium  compound  on  the  charged  plate  of  a  gold- 
leaf  electroscope.  Indications  of  the  existence  of  the  element 
uranium  were  given  by  Klaproth  in  1789,  and  the  discharging 
property  of  this  substance  could  not  fail  to  have  been  noted  if 
it  had  been  placed  near  a  charged  electroscope.  It  would  riot 
have  been  difficult  to  deduce  that  the  uranium  gave  out  a  type 
of  radiation  capable  of  passing  through  metals  opaque  to  ordi- 
nary light.  The  advance  would  probably  have  ended  there,  for 
the  knowledge  at  that  time  of  the  connection  between  electricity 
and  matter  was  far  too  meagre  for  an  isolated  property  of  this 
kind  to  have  attracted  much  attention. 

It  is  not  necessary,  however,  to  go  so  far  back  to  illustrate 
the  great  influence  that  the  electrical  discoveries  in  the  allied 
field  of  discharge  of  electricity  through  gases  have  had  on  the 
rapid  development  of  radioactivity.  If  the  discovery  had  been 
made  even  a  decade  earlier,  the  advance  must  necessarily  have 
been  much  slower  and  more  cautious.  At  that  time  the  possi- 
bility of  the  existence  of  radiations  capable  of  penetrating 
matter  opaque  to  light  had  not  even  been  considered,  and  the 
true  nature  of  the  cathode  rays  was  still  a  matter  for  conjec- 
ture. The  character  of  the  radiations  from  radioactive  matter, 
as  we  know  them  to-day,  could  only  have  been  deduced  after 
a  long  and  laborious  series  of  researches,  for  not  only  would  the 
experimenter  have  had  no  guidance  from  analogy,  but  he  must 
of  necessity  have  developed  the  methods  of  attack  de  novo  under 
difficult  conditions.  It  would  have  been  necessary,  also,  to  have 
examined  in  detail  the  nature  of  the  discharging  action  of  the 
rays,  for  on  this  is  based  the  most  important  method  of  measure- 
ment in  radioactivity. 

Let  us  now  examine  the  conditions  that  existed  during  the 
actual  development  of  the  subject.  We  have  seen  that  the 
mechanism  of  the  conduction  of  electricity  through  gases  had 
been  developed  primarily  by  a  study  of  the  conductivity  of 
gases  exposed  to  X-rays,  and  of  the  discharge  of  electricity  in 

2 


18  RADIOACTIVE   TRANSFORMATIONS 

a  vacuum  tube.  The  knowledge  thus  obtained  was  directly 
applied  to  the  corresponding  ionization  produced  by  the  radia- 
tions from  active  matter  and  served  as  a  foundation  for  the  elec- 
tric method  of  measurement  which  has  been  utilized  as  a  rapid 
quantitative  means  of  radioactive  analysis.  When  the  fi  rays 
of  radium  were  found  to  be  deflected  in  a  magnetic  field  in  the 
same  way  as  the  cathode  rays,  it  was  only  necessary,  in  order 
to  prove  their  identity,  to  adopt  the  methods  which  had  been 
familiar  to  science  for  several  years.  In  a  similar  way,  the 
behavior  of  the  non-deflectable  7  rays  was  directly  compared 
with  the  known  properties  of  X-rays,  while  the  a  rays  were 
found  to  be  analogous  in  some  respects  to  the  canal  rays  of 
Goldstein,  which  had  been  shown  previously  by  Wien  to  be 
deflected  by  a  magnetic  and  electric  field. 

The  influence  of  the  ionization  theory  on  the  development  of 
radioactivity  has  been  equally  marked  in  other  directions.  The 
determination  of  the  charge  carried  by  an  ion  has  been  of  the 
greatest  utility  in  determining  the  order  of  magnitude  of  the  proc- 
esses occurring  in  radioactive  matter.  These  data  have  been 
of  great  value  in  determining  the  number  of  a  and  /3  particles 
emitted  by  radium  and  in  suggesting  the  probable  amount  of 
emanation  and  of  helium  liberated  from  it.  Such  calculations 
have  enabled  us  to  fix  with  some  certainty  the  rate  at  which 
radium  and  the  other  radioactive  bodies  suffer  disintegration, 
and  also  to  determine  beforehand  the  magnitude  of  many  physi- 
cal and  chemical  quantities,  and  have  thus  indirectly  suggested 
the  methods  of  attack  necessary  to  solve  the  various  problems 
which  have  arisen. 

The  fortunate  combination  of  events  in  the  history  of  radio- 
activity is  strikingly  illustrated  by  the  discovery  that  helium  is 
evolved  by  the  radium  emanation.  This  rare  gas  has  a  dramatic 
history,  for  its  presence  was  first  observed  in  the  sun  by  Janssen 
and  Lockyer  in  1868;  but  it  was  not  until  1895  that  its  pres- 
ence in  the  rare  mineral  clevite  was  observed  by  Ramsay.  An 
examination  of  its  physical  and  chemical  properties  had  hardly 
been  completed  when  Ramsay  and  Soddy,  guided  by  the  disin- 
tegration theory,  made  an  examination  of  the  gases  liberated 


HISTORICAL   INTRODUCTION  19 

from  radium,  and  discovered  that  helium  was  a  product  of  the 
transformation  of  radium.  If  helium  had  not  a  short  time 
before  been  found  in  radioactive  minerals,  it  is  safe  to  say  that 
this  most  striking  property  of  radium  of  producing  helium 
would  have  long  remained  hidden. 

While  the  ionization  theory  has  played  a  prominent  part  in 
the  extension  of  our  knowledge  in  radioactivity,  the  benefits 
have  not  been  altogether  one-sided,  for  the  results  obtained 
from  a  study  of  radioactivity  have  greatly  assisted  in  extending 
and  confirming  the  ionization  theory.  It  has  placed  in  the  hands 
of  the  experimenter  a  constant  and  powerful  source  of  ionizing 
radiation  in  place  of  a  variable  source  like  X-rays,  and  this  has 
been  of  great  service  in  obtaining  accurate  data.  In  addition, 
the  results  of  Kaufmann  on  the  variation  of  the  mass  of  the  /3 
particle  from  radium  with  its  velocity,  have  been  an  important 
factor  in  confirming  and  extending  our  conception  of  electrons. 

Examples  of  this  kind  could  readily  be  multiplied,  but  suffi- 
cient have  been  given  to  illustrate  the  close  connection  that  has 
existed  and  still  exists  between  these  two  distinct  lines  of  in- 
vestigation, and  the  influence  which  each  has  exerted  upon  the 
development  of  the  other. 

RADIATIONS  FROM  ACTIVE  BODIES 

A  brief  summary  will  now  be  given  of  the  chief  properties 
and  nature  of  the  a,  /3,  and  7  rays  emitted  from  radioactive 
bodies.  All  three  types  of  rays  possess  in  common  the  proper- 
ties of  acting  on  a  photographic  plate,  of  exciting  phosphores- 
cence in  certain  substances,  and  of  discharging  electrified  bodies. 
The  three  types  of  rays  can  be  distinguished  from  one  another 
by  their  difference  in  penetrating  power  and  by  the  effect  upon 
them  of  a  magnetic  or  electric  field.  The  a  rays  are  completelv 
stopped  by  a  layer  of  aluminium  about  .05  mm.  in  thickness; 
the  greater  part  of  the  (3  rays  by  5  mms.  of  aluminium,  while 
a  thickness  of  at  least  50  cms.  of  aluminium  would  be  required 
to  absorb  most  of  the  7  rays.  The  relative  penetrating  power  of 
the  three  types  of  rays  is  thus  about  in  the  ratio  1  :  100  :  10000. 
It  must  be  borne  in  mind,  however,  that  this  is  an  average  value, 


20  RADIOACTIVE   TRANSFORMATIONS 

for   each   type  of  radiation   is   complex   and   consists    of  rays 
unequally  absorbed  by  matter. 

The  a  rays  consist  of  positively  charged  particles  projected 
with  a  velocity  of  about  twenty  thousand  miles  per  second. 
The  apparent  mass  of  the  a  particle  is  about  twice  that  of  the 
hydrogen  atom.  Although  the  magnetic  deflection  of  these  rays 
has  only  so  far  been  observed  in  the  case  of  active  substances 
like  radium  and  polonium,  there  can  be  little  doubt  that  the 
a  rays  from  the  other  radioactive  bodies  are  of  a  very  similar 
nature. 

The  &  rays  consist  of  negatively  charged  particles  projected 
with  great  velocities.  Their  apparent  mass  is  about  roV o  of  that 
of  the  hydrogen  atom,  and  they  are  identical  in  all  respects 
except  velocity  with  the  cathode-ray  particles  set  free  in  a 
vacuum  tube. 

In  the  case  of  radium,  the  ft  particles  are  projected  with  a 
wide  range  of  velocity,  the  maximum  approaching  very  closely 
to  the  velocity  of  light,  ft  rays  are  also  given  out  by  uranium, 
thorium,  and  actinium. 

The  7  rays  are  not  deflected  by  a  magnetic  or  electric  field, 
and  closely  resemble  in  general  properties  the  very  penetrating 
X-rays  produced  in  a  hard  vacuum  tube.  According  to  present 
views,  the  7  rays  must  thus  be  supposed  to  be  a  t}Tpe  of  wave 
motion  in  the  ether,  consisting  probably  of  pulses  set  up  as 
a  result  of  the  expulsion  of  ft  particles.  Only  those  active 
substances  which  emit  ft  rays  give  rise  to  7  rays.  The  7  rays 
are  emitted  from  uranium,  thorium,  radium,  and  actinium,  but 
the  rays  from  uranium  and  actinium  have  not  such  a  marked 
power  of  penetration  as  the  rays  from  thorium  and  radium. 

Each  of  these  three  primary  types  of  rays  falling  upon  matter 
gives  rise  to  secondary  rays.  In  the  case  of  the  a  rays  the 
secondary  radiation  consists  of  negatively  charged  particles 
(electrons)  projected  at  velocities  comparatively  small  com- 
pared with  those  of  the^cparticles  themselves.  The  secondary 
rays  arising  from  the  ft  and  7  rays  consist  in  part  of  electrons 
projected  with  considerable  velocity.  These  secondary  rays  in 
turn  produce  tertiary  rays,  and  so  on. 


HISTORICAL   INTRODUCTION 


21 


If  a  strong  magnetic  field  is  applied  at  right  angles  to  a 
pencil  of  a,  /3,  and  7  rays,  the  three  types  of  rays  are  sepa- 
rated from  each  other.  This  is  shown  in  Fig.  1,  where  the 
magnetic  field  is  acting  downwards,  perpendicular  to  the  plane 
of  the  paper.  The  ft  rays 
are  bent  to  the  right,  the  a 
rays  to  the  left,  while  the 
7  rays  are  unaffected.  The 
ft  rays  consist  of  particles 
having  unequal  velocities 
and  consequently  traversing 
circular  orbits  of  different, 
radii  of  curvature.  The 
magnetic  deflection  of  the 
a  rays  compared  with  that 
of  the  ft  rays  is  much  exag-  ' 
gerated  in  the  figure.  FlG-  l- 

The  relative  mass,  veloci-        Separation  of  radium  rays  by  the  action 

ty,and  kinetic  energy  of  the    of  a  magnetic  field- 

average  a  and  ft  particles  are    shown   in    Fig.    2,    where    the 

volume  of  a  sphere  represents  mass  and  energy,  and  the  length 

of  line  represents  velocity. 

It  will  be  seen  from  this  illustration  that  although  the  average 

ft  particle  has  a  much  higher  velocity  than  the  average  a  parti- 
cle, its  energy  of  mo- 
tion on  account  of  its 
relatively  small  mass 
is  much  less  than 
that  of  the  average 
^particle.  This  re- 
sult is  in  accordance 
with  the  observed 


MASS          VELOCITY      ENERGY 

4 

o 

— 

® 

P 

• 

( 

0 

FIG.  2. 


result  that  the  ioni- 
zation    and    heating 

effect  produced  by  the  a  particle  are  much  greater  than  for  the 

ft  particle. 

The  writer  has  recently  shown  that  one  gram  of  radium  in 


22  RADIOACTIVE   TRANSFORMATIONS 

radioactive  equilibrium  emits  about  7  x  1010  ft  particles  and 
about  2.5  X  10n  a  particles  per  second.  Four  a  particles  are 
thus  expelled  from  radium  for  each  ft  particle. 

RADIOACTIVE  SUBSTANCES 

Below  is  given  a  list  of  the  radioactive  substances  which  have 
so  far  been  separated.  The  nature  of  the  radiations,  and  the 
presence  or  absence  of  an  emanation  is  also  noted.  The  "  period  " 
of  the  emanation  denotes  the  time  required  for  its  activity  to 
fall  to  half  value. 

Uranium  :  a,  (3,  and  y  rays,  but  no  emanation. x 

Thorium :  a,  ft,  and  y  T&VS  ;  an  emanation,  period  54  seconds. 

Radium  :  a,  ft,  and  y  rays ;  an  emanation,  period  3.8  days. 

c  mmm  )       „  ^^  g     an  emana^iou   period  3.9  seconds. 

Emanium  ) 

v  only  a  rays  :  no  emanation. 
Radiotellurmm  ) 

Radiolead  (some  time  after  preparation)  :  a,  ft,  and  y  rays,  but 
no  emanation. 

These  substances,  with  the  exception  of  polonium,  continue 
to  radiate  for  long  periods  of  time.  In  addition,  there  are  a 
number  of  radioactive  products  arising  from  each  radioelement 
which  have  a  comparatively  short  radioactive  life.  These  prod- 
ucts are  intrinsically  as  important  as  the  more  permanently 
active  substances,  and  have  an  equal  right  to  be  called  elements. 
On  account  of  the  rapidity  of  their  transformation,  they  exist  in 
extremely  small  quantity  in  pitchblende,  and  are  never  likely 
to  be  obtained  in  sufficient  quantity  to  be  examined  by  ordinary 
chemical  methods.  Polonium  and  radiotellurium,  which  con- 
tain the  same  radioactive  constituent,  differ  from  the  other 
substances  in  emitting  only  a  rays.  In  regard  to  their  length  of 
life,  they  occupy  an  intermediate  position  between  the  rapidly 
transformed  products  like  the  emanation,  and  a  very  slowly 
changing  substance  like  radium.  The  activity  of  radiotellurium 
falls  to  half  value  in  about  140  days,  while  the  corresponding 
time  for  radium  is  about  1300  years. 


HISTORICAL  INTRODUCTION  23 

With  the  exception  of  uranium,  thorium,  and  radium,  none  of 
these  substances  have  been  sufficiently  purified  to  determine 
their  atomic  weight  or  spectrum.  It  seems  likely,  however, 
that  actinium  will  prove  to  be  an  element  at  least  as  active  as 
radium.  It  will  also  be  shown  later  that  radiotellurium  and 
radiolead  in  a  pure  state  should  be  much  more  active,  weight 
for  weight,  than  radium. 

The  activity  of  a  given  substance  which  emits  a  rays  de-> 
pends  on  the  number  of  a  particles  shot  out  per  second,  and  } 
this,  for  equal  weights,  is  inversely  proportional  to  the  "  period  " 
of  that  substance.  For  example,  the  actinium  emanation  whose 
period  is  3.9  seconds  must  be,  weight  for  weight,  at  least  one 
thousand  million  times  as  active  as  radium.  It  is  on  account 
of  their  enormous  activity  and  consequent  rapidity  of  transforma- 
tion that  such  substances  can  never  be  obtained  in  sufficient 
quantity  for  chemical  analysis.  It  is  only  the  more  slowly 
changing  substances  like  radium,  radiolead,  and  radiotellurium 
that  collect  in  sufficient  quantity  in  pitchblende  to  be  chemi- 
€ally  isolated  in  appreciable  quantity. 

It  will  also  be  shown  later  that  the  radiations  emitted  from 
uranium,  radium,  thorium,  and  actinium  arise  only  in  part  from 
the  primary  active  substance  itself.  The  /3  and  7  rays  in  all 
-cases  are  emitted  from  the  products  of  the  transformation  of 
these  elements.  These  are  mixed  with  the  parent  substance 
and  add  their  radiations  to  it. 

METHODS  OF  MEASUREMENTS 

There  are  three  general  properties  of  the  rays  from  radio- 
active substances  which  have  been  utilized  for  the  purpose  of 
measurements,  depending  on  (1)  the  action  of  the  rays  on  a 
photographic  plate,  (2)  the  phosphorescence  excited  in  certain 
crystalline  substances,  (3)  the  ioriization  produced  by  the  rays 
in  a  gas.  Of  these  the  phosphorescent  method  is  limited  to 
substances  like  radium,  actinium,  and  polonium  which  emit  very 
intense  radiations.  The  a,  /3,  and  7  rays  all  produce  a  marked 
luminosity  in  the  platinocyanides  and  in  the  mineral  willemite 
(zinc  silicate).  The  mineral  kunzite  responds  mainly  to  the  ]3 


24  RADIOACTIVE   TRANSFORMATIONS 

and  7  rays,  while  Sidot's  blende  (crystalline  zinc  sulphide)  re- 
sponds mainly  to  the  a  rays.  Besides  these  there  are  a  large 
number  of  substances  in  which  a  more  or  less  feeble  luminosity 
is  excited  by  the  rays.  The  property  of  the  a  rays  of  producing 
scintillations  on  a  screen  covered  with  zinc'  sulphide  is  especially 
interesting,  and  it  has  been  found  possible  by  this  method  to  de- 
tect the  a  rays  emitted  by  feebly  active  substances  like  uranium, 
thorium,  and  pitchblende.  Screens  of  zinc  sulphide  have  been 
used  as  an  optical  method  for  demonstrating  the  presence  of  the 
emanations  from  radium  and  actinium.  Speaking  generally, 
the  phosphorescent  method,  while  very  interesting  as  an  optical 
means  for  examining  the  rays,  is  very  limited  in  its  application 
and  is  only  roughly  quantitative. 

The  photographic  method  proved  of  great  service  in  the  early 
development  of  radioactivity,  but  has  gradually  been  displaced  by 
the  electric  method  as  quantitative  determinations  have  become 
more  and  more  necessary.  It  has  proved  of  special  utility  in 
examination  of  the  curvature  of  the  path  of  the  rays  in  magnetic 
and  electric  fields.  On  the  other  hand,  it  does  not  readily  lend 
itself  to  quantitative  comparisons  and  is  very  limited  in  its  appli- 
cation. In  the  case  of  feebly  active  substances  like  uranium 
and  thorium,  long  exposures  are  necessary  to  produce  much 
photographic  effect.  It  cannot  be  utilized  to  follow  the  rapid 
changes  of  activity  which  are  exhibited  by  many  radioactive 
products,  and  is  not  sufficiently  sensitive  to  detect  the  presence 
of  rays  which  are  readily  observed  by  the  electric  method. 

The  development  of  the  subject  of  radioactivity  has  largely 
depended  on  the  electric  method  of  measurement,  which  is  uni- 
versally applicable,  and  far  transcends  in  delicacy  either  of  the 
other  two  methods.  It  readily  lends  itself  to  rapid  quantitative 
measurements,  and  can  be  applied  to  all  the  types  of  radiation 
which  possess  the  ionizing  property. 

This  method  is,  as  we  have  seen,  based  on  the  property  of 
the  a,  &  and  y  rays  of  producing  charged  carriers  or  ions  in  the 
volume  of  the  gas  traversed  b^  the  radiations.  Suppose  that  a 
layer  of  radioactive  substance — urahTtrm^  forexample  —  is  placed 
on  the  lower  of  two  insulated  parallel  plates,  A  and  B.  (Fig.  3). 


HISTORICAL   INTRODUCTION  25 

The  gas  between  the  plates  is  ionized  at  a  constant  rate  by  the 
radiations,  and  there  results  a  distribution  of  positive  and  nega- 
tive ions  in  the  volume  of  air.  If  no  electric  field  is  acting,  the 
number  of  ions  does  not  increase  indefinitely,  but  soon  reaches 
a  maximum,  when  the  rate  of  production  of  fresh  ions  by  the 
radiations  exactly  compensates  for  the  decrease  in  the  number 
due  to  the  recombination  of  the  positive  and  negative  ions. 
This  latter  effect  will  obviously  tend  to  take  place  when  the 
positive  and  negative  ions  in  the  course  of  their  movement  come 
within  the  sphere  of  one  another's  attraction.  Suppose  now 
that  the  plate  A  is  kept  charged  to  a  constant  potential  V,  and 


B     - 


e/t*Ttt 

FIG.  3. 

that  the  rate  at  which  B,  initially  at  zero  potential,  gains  an 
electric  charge  is  determined  by  a  suitable  measuring  instru- 
ment, for  example,  a  quadrant  electrometer. 

Under  the  influence  of  the  electric  field,  the  positive  ions 
travel  to  the  negative  plate  and  the  negative  ions  to  the  positive. 
There  is  consequently  a  current  through  the  gas,  and  the  plate 
B  and  its  connections  acquire  a  positive  charge.  The  rate  at 
which  the  plate  B  rises  in  potential  is  a  relative  measure  of  the 
current  through  the  gas.  When  V  has  a  small  value,  the  cur- 
rent is  small,  but  gradually  increases  with  rise  of  V,  until  a 
stage  is  reached  where  the  current  increases  very  slightly  for 
a  large  increment  of  the  value  of  V.  The  relation  between  the 


26  RADIOACTIVE   TRANSFORMATIONS 

current  and  the  applied  voltage  is  seen  in  Fig.  4.  The  shape 
of  this  curve  receives  a  simple  explanation  on  the  ionization 
theory.  The  ions  move  with  a  velocity  proportional  to  the 
strength  of  the  electric  field.  In  a  weak  field  there  is  thus 
a  slow  movement  of  the  positive  and  negative  ions  past  one 
another.  A  large  proportion  of  the  ions  have  time  to  recom- 
bine  before  they  reach  the  electrodes,  and  the  current  observed 
through  the  gas  is  consequently  small.  As  the  voltage  in- 
creases, the  velocity  of  the  ions  increases,  and  there  is  less  time 
for  recombination.  Finally,  in  a  strong  field  practically  all  the 


VOLTS. 

FIG.  4. 
Typical  saturation  curve  for  an  ionized  gas. 

ions  are  swept  to  the  electrodes  before  any  appreciable  recom- 
bination can  occur.  The  maximum  or  "saturation"  current 
through  the  gas  is  then  a  measure  of  the  charge  carried  by  the 
ions  produced  per  second  by  the  radiation,  i.  e.,  it  is  a  measure 
of  the  total  rate  of  production  of  ions. 

The  term  "saturation,"  which  was  applied  initially  from  the 
resemblance  of  the  current-voltage  curve  to  the  magnetization 
curve  for  iron,  is  not  very  suitable,  but  has  come  into  use  as  a 
convenient  though  inaccurate  method  of  expressing  an  experi- 
mental fact. 

Other  conditions  being  the  same,  the  voltage  required  to 
produce  saturation  increases  with  the  intensity  of  the  ioniza- 


VER31TY   V 


HISTORICAL  INTRODUCTION  27 

tion,  i.  <?.,  with  increase  in  activity  of  the  substance  under  ex- 
amination. Increase  of  the  distance  between  the  plates  lowers 
the  value  of  the  electric  field  and  increases  the  distance  over 
which  the  ions  move.  Both  of  these  conditions  tend  to  increase 
the  voltage  to  be  applied  in  order  to  give  saturation. 

It  is  found  experimentally  that  for  parallel  plates  not  more 
than  3  or  4  cms.  apart,  300  volts  is  sufficient  to  produce  ap- 
proximate saturation,  using  substances  whose  activities  are  not 
greater  than  1000  times  that  of  uranium.  For  intensely  ac- 
tive substances  like  radium,  in  order  to  produce  saturation 
at  all,  the  plates  must  be  close  together  and  a  high  voltage 
applied. 

The  essential  condition  for  quantitative  comparisons  by  the 
electric  method  depends  on  the  measurement  of  the  saturation 
current,  for  this  is  a  measure  of  the  total  number  of  ions  pro- 
duced per  second  in  the  volume  of  gas  under  consideration. 

The  electric  method  can  be  used  with  accuracy  to  compare 
the  relative  activity  of  substances  which  emit  identically  the 
same  rays,  but  differ  only  in  the  intensity  of  the  radiations.  It 
serves,  for  example,  to  determine  accurately  the  rate  at  which 
simple  products  like  the  emanations  lose  their  activities. 

Unless  other  factors  are  taken  into  consideration,  the  electric 
method  cannot  be  directly  used  to- compare  the  relative  inten- 
sity of  different  types  of  radiation.  For  example,  the  relative 
saturation  currents  produced  by  the  a  and  ft  rays,  emitted  from 
a  thick  layer  of  uranium,  under  the  conditions  shown  in  Fig.  3, 
cannot  be  used  as  a  direct  comparison  of  the  intensity  of  the 
two  types  of  radiations,  for  on  account  of  their  difference  in 
penetrating  power,  a  much  smaller  fraction  of  the  total  energy 
of  the  issuing  fi  rays  is  absorbed  in  producing  ions  between  the 
plates  than  in  the  case  of  the  more  easily  absorbed  a  rays.  Be- 
fore such  comparisons  of  ionization  currents  can  be  used  to  . 
measure  the  relative  amounts  of  energy  of  the  two  types  of 
rays,  the  relative  penetrating  and  ionizing  powers  of  the  types 
of  radiation  must  be  accurately  known.  The  main  province  of 
the  electric  method,  however,  lies  in  the  determination  of  the 
variations  in  the  activity  of  a  body  which  emits  rays  all  of  one 


28 


RADIOACTIVE   TRANSFORMATIONS 


kind,  and  in  this  it  has  proved  of  great  value,  and  has  yielded 
results  of  considerable  accuracy. 

A  variety  of  methods  have  been  employed  to  measure  the 
ionization  currents  produced  by  the  radiations.  If  a  very  active 
substance  is  under  examination,  a  sensitive  galvanometer  may 
be  used  to  measure  the  saturation  current.  With  slight  modifi- 
cations, the  gold-leaf  electroscope  has  proved  an  accurate  and 
reliable  means  of  measurement,  and  has  played  a  prominent 

part  in  the  development 
of  radioactivity.  Various 
types  of  the  instrument 
have  been  employed.  A 
simple  form  which  I  have 
found  very  convenient  in 
comparisons  of  activities  is 
shown  in  Fig.  5.  The  ac- 
tive material  is  placed  on 
the  lower  .plate,  A,  which 
is  mounted  on  a  slide  so 
that  it  can  be  easily  moved 
out  to  place  the  radioactive 
»  material  in  position.  The 
upper  plate,  B,  placed  about 


FIG.  5. 

Simple  electroscope  for  comparing 
o-ray  activities. 


3  cms.  above  A,  is  connected 
with  a  rod,  R,  which  is  rig- 
idly supported  by  a  cross 
rod,  TT,  on  two  insulating 
sulphur  rods,  SS.  The  aluminium  or  gold  leaf  is  connected 
with  the  upper  part  of  the  rod  R.  The  rod  C,  when  con- 
nected with  a  suitable  voltage,  serves  to  charge  the  electroscope 
system. 

The  movement  of  the  gold  leaf  is  observed  through  glass  or 
mica  windows  by  means  of  a  low -power  microscope  provided 
with  a  micrometer  scale  in  the  eyepiece.  The  lower  plate,  A, 
and  the  external  case,  PP,  are  connected  to  earth. 

By  suitably  adjusting  the  length  of  the  gold  leaf  and  the 
position  of  the  boundaries,  the  time  taken  for  the  gold  leaf  to 


HISTORICAL   INTRODUCTION 


29 


pass  over  a  definite  number  of  divisions  of  the  scale  in  the  eye- 
piece may  be  made  nearly  constant  over  a  considerable  range. 
The  radioactive  material,  placed  in  a  metal  or  other  conducting 
vessel,  is  put  in  position.  The  electroscope  is  then  charged 
and  the  time  taken  for  the  gold  leaf  to  pass  over  a  fixed  part  of 
the  scale  is  observed.  This  must  be  corrected  for  the  natural 
leak  of  the  instrument,  which  is  determined  before  the  introduc- 
tion of  the  active  material.  This  natural  leak  may  be  due  in 
part  to  a  slight  leakage  over  the  sulphur 
supports,  or  more  generally  to  a  weak 
activity  of  the  walls  of  the  electroscope. 
All  substances  are  slightly  active,  and 
this  activity  is  often  increased  by  radio- 
active contamination  from  the  presence 
of  radium  and  other  emanations.  Two 
or  three  hundred  volts  is  sufficient  to 
charge  the  electroscope,  and  this  insures 
saturation  over  the  greater  part  of  the 
range  provided  that  the  active  material 
does  not  cause  the  electroscope  to  lose 
its  charge  in  less  than  two  or  three 
minutes. 

In  this  way  measurements  can  be 
made  with  rapidity  and  certainty.  An 
accuracy  of  one  per  cent  can  readily  be 
obtained,  and  with  care  the  precision  of 
measurement  may  be  made  still  greater. 
The  great  advantages  of  this  type  of  instrument  are  its  simplic- 
ity, portability,  and  comparative  ease  of  construction.  Such  an 
instrument,  if  standardized  by  a  constant  source  of  radiation 
like  uranium,  is  very  suitable  for  determining  the  variation  of 
activity  of  substances,  which  change  very  slowly  with  time. 

A  modification  of  this  electroscope,  first  used  by  C.  T.  R. 
Wilson,  can  be  utilized  to  -measure  extraordinarily  minute 
ionization  currents.  The  construction  of  the  apparatus  is  seen 
in  Fig.  6. 

A  clean  metal  vessel,  preferably  of  brass,  of  about  one  litre 


FIG.  6. 

Electroscope  for  compar- 
ing £  and  7  ray  activities 
and  for  measurement  of  very 
weak  activities. 


30  RADIOACTIVE  TRANSFORMATIONS 

capacity,  has  a  gold  leaf,  L,  attached  to  a  rod,  R,  insulated  by 
a  sulphur  or  amber  bead,  S,  inside  the  vessel.  This  is  charged 
by  a  movable  rod,  C,  or  by  a  magnetic  device.  After  charg- 
ing, the  upper  rod,  P,  is  connected  to  the  case  of  the  instrument 
and  to  earth.  In  special  cases,  if  extremely  minute  currents 
are  to  be  measured,  the  rod  P  is  kept  connected  with  a  source  of 
potential  slightly  greater  than  the  potential  of  the  electroscope 
system.  This  insures  that  there  is  no  leak  of  the  charge  across 
the  sulphur  support. 

The  movement  of  the  gold  leaf  is  observed  as  before  with  a 
microscope  having  a  micrometer  eyepiece.  The  great  advan- 
tage of  this  instrument  lies  in  the  fact  that  the  apparatus  can 
be  hermetically  closed.  The  rate  of  leak  observed  must  then 
be  entirely  due  to  the  ionization  in  the  interior  of  the  vessel 
and  be  independent  of  external  electrostatic  disturbances. 

An  instrument  of  this  kind  is  very  useful  for  comparing 
y8  and  7  ray  activities.  For  the  former,  the  base  of  the 
electroscope  is  removed,  and  replaced  by  a  sheet  of  aluminium, 
about  .1  mm.  in  thickness,  which  completely  stops  the  a  rays, 
but  allows  the  /3  rays  to  pass  through  with  little  absorption. 
For  measurements  of  the  7  rays,  the  vessel  is  placed  on  a  lead 
plate  about  5  rnms.  thick,  and  the  active  material  placed  beneath. 
The  /8  rays  are  completely  absorbed  by  this  thickness  of  lead, 
and  the  ionization  of  the  vessel  is  then  due  to  the  more  penetrat- 
ing 7  rays. 

The  most  convenient  general  method  of  measurement  depends 
on  the  use  of  a  quadrant  electrometer.  A  very  convenient  and 
useful  type  of  electrometer  for  radioactive  and  other  work  has 
been  designed  by  Dolezalek.1  The  general  construction  of  the 
instrument  is  seen  in  Fig.  7. 

The  four  quadrants  are  mounted  on  amber  or  sulphur  sup- 
ports. A  very  light  needle,  N,  is  made  out  of  silvered  paper 
and  is  suspended  by  a  fine  quartz  fibre  or  phosphor-bronze  strip. 
The  needle  is  charged  to  a  potential  of  100  to  300  volts.  If 
a  quartz  suspension  is  used,  this  is  done  by  lightly  touching  the 
metallic  support  of  the  needle  with  a  wire  connected  with  the 

1  Dolezalek:  Instmmentenkunde,  p.  345,  1901. 


HISTORICAL   INTRODUCTION 


31 


source  of  potential.  It  is  often  more  convenient  to  use  a  fine 
phosphor-bronze  suspension.  The  needle  may  then  be  directly 
connected  with  one  terminal  of  a  battery  the  other  pole  of  which 
is  earthed,  and  its  potential  kept  constant.  By  the  use  of  a  fine 
quartz  suspension,  the  sensibility  of  the  instrument,  i.  e.,  the 
number  of  millimetre  divisions,  passed  over  by  the  spot  of  light 


FIG.  7. 
Dolezalek  electrometer. 

on  the  scale  for  an  application  of  a  difference  in  potential  of  one 
volt  between  the  quadrants,  may  be  made  very  great.  A  sen- 
sibility of  10,000  millimetre  divisions  per  volt  is  not  uncommon. 
Unless,  however,  a  very  small  current  is  to  be  measured,  it  is 
not  advisable  to  work  with  a  sensibility  greater  than  1000  divi- 
sions per  volt,  and  200  divisions  is  often  sufficient. 


32 


RADIOACTIVE   TRANSFORMATIONS 


The  quadrant  electrometer  is  essentially  an  instrument  for 
measuring  the  potential  of  the  conductor  with  which  it  is 
connected,  but  is  indirectly  used  in  radioactivity  to  measure 
ionization  currents.  The  capacity  of  the  electrometer  and  its 
connections  remains  sensibly  constant  with  the  movement  of 
the  needle,  and  the  rate  at  which  the  spot  of  light  moves  over 
the  scale  is  a  measure  of  the  rate  of  rise  of  potential  of  the  elec- 
trometer system.  This  serves  as  a  measure  of  the  ionization 
current  between  the  electrodes  of  the  testing  vessel. 

The  general  arrangement  for  measurement  is  shown  in  Fig.  8. 
The  active  material  is  placed  on  the  lower  of  two  parallel  in- 
sulated plates,  A  and  B.  The  plate  A  is  connected  with  one 


CLKTRQMfTCR, 


TfaTIN*     VCSSfL. 


CAQTH. 


FIG.  8. 
Method  of  use  of  electrometer  for  comparing  activities. 

terminal  of  a  battery  of  suitable  potential,  and  the  plate  B  is 
connected  through  a  key,  K,  with  one  pair  of  quadrants  of  the 
electrometer.  When  not  in  use  the  key  connects  the  quadrants 
and  its  connections  to  earth.  When  a  measurement  is  required, 
the  earth  connection  is  quietly  but  quickly  broken  by  means  of 
some  mechanical  or  electro-magnetic  device.  The  plate  B  and 
its  connections  rise  in  potential,  and  this  is  indicated  by  the 
movement  of  the  spot  of  light  over  the  scale.  The  time  taken 
for  the  spot  of  light  to  move  over  a  definite  distance  of  the  scale 
is  then  observed,  and  the  number  of  divisions  passed  over  per 
second  serves  as  a  comparative  measure  of  the  ionization  current 
through  the  gas. 

When  the  movement  of  the  spot  of  light  becomes  too  rapid 
for  accurate  observation,  an  additional  capacity  in  the  form  of 


HISTORICAL   INTRODUCTION  33 

an  air  or  mica  condenser  is  added  to  the  electrometer  system 
and  the  rate  of  movement  is  reduced  to  that  required. 

Proceeding  in  this  way,  activities  can  readily  be  compared 
over  a  very  wide  range.  The  magnitude  of  the  current  that 
can  be  measured  is  only  limited  by  the  capacity  of  the  con- 
densers available  and  the  voltage  of  the  battery,  which  must 
be  sufficient  to  produce  saturation  in  the  testing  vessel. 

This  use  of  electroscopes  and  electrometers  to  compare  activi- 
ties thus  depends  on  the  rate  of  angular  movement  of  the  mov- 
ing system.  By  a  suitable  arrangement,  an  electrometer  may 
be  used  as  a  direct  reading  instrument  for  measuring  current  in 
the  same  way  as  a  galvanometer. 

Suppose  that  the  electrometer  system  is  connected  to  earth 
through  a  very  high  resistance,  which  obeys  Ohm's  law.  When 
the  earth  connection  of  the  quadrants  is  broken,  the  plate  B 
(Fig.  8)  rises  in  potential  until  the  supply  of  electricity  to 
B  exactly  compensates  for  the  loss  of  electricity  by  discharge 
through  the  high  resistance.  Since  the  deflection  of  an  elec- 
trometer needle  is  proportional  to  the  voltage  applied,  the  spot 
of  light  will  thus  move  from  rest  to  a  steady  position,  and  this 
deflection  is  proportional  to  the  ionizing  current  in  the  testing 
vessel. 

For  measurements  of  this  character,  the  resistance  used  must 
be  of  the  order  10  n  ohms.  The  main  drawback  of  the  method 
is  the  difficulty  of  obtaining  suitable  resistances  of  this  charac- 
ter which  are  at  the  same  time  free  from  polarization  and  obey 
Ohm's  law.  The  principle  of  this  method  has  been  employed  in 
experiments  by  Dr.  Bronson l  in  the  laboratory  of  the  writer. 

Such  an  arrangement  is  especially  suitable  fer  following  with 
accuracy  rapid  variations  of  activity.  The  deflection  is  inde- 
pendent of  the  capacity  of  the  electrometer  and  connections,  and 
measurements  can  be  made  quickly  and  accurately  over  a  wide 
range. 

Some  of  the  types  of  testing  vessels  suitable  for  comparisons 
of  activity  by  the  electrometer  method  are  shown  in  Figs.  9 
and  10. 

1  Bronson  :  Amer.  Journ.  Science,  July,  1905  ;  Phil.  Mag.,  Jan.,  1906. 

3 


34 


RADIOACTIVE   TRANSFORMATIONS 


The  active  material  is  placed  on  the  lower  of  two  parallel 
plates,  A  and  B,  in  a  closed  vessel  (Fig.  9).  The  insulated 
plate,  B,  is  attached  by  ebonite  rods  to  the  case  of  the  apparatus 


U.VJSICCI 

oTneier 

\ 

B 

Active  Material 
\                        I        A 

1 

N 

I 

J 

](©— Wo  Battery 


FIG.  9. 

Parallel  plate  testing  vessel. 

which  is  connected  with  earth,  so  that  there  can  be  no  direct 
conduction  leak  from  the  battery  to  B. 

In  Fig.  10  is  shown  a  cylindrical  testing  vessel,  B,  suitable 
for  comparison  of  the  activities  obtained  on  wires  or  rods.     The 


tarth 


Earth 

FIG.  10. 

Cylindrical  testing  vessel  with  guard  ring  for  comparing  activities 
on  wires  or  rods. 

inner  active  electrode,  A,  passes  through  an  ebonite  cork. 
The  conduction  leak  across  the  ebonite  is  avoided  by  use  of 
the  guard  ring  principle,  a  metal  cylinder,  CO',  connected  with 
earth  dividing  the  ebonite  cork  into  two  parts.  In  this  case,  the 
ebonite  has  only  to  be  insulated  sufficiently  for  the  small  rise 


HISTORICAL   INTRODUCTION  35 

of  potential  required  to  cause  a  suitable  deflection  of  the  elec- 
trometer needle.  The  adoption  of  the  guard  ring  principle  is 
advisable  in  all  cases  in  order  to  get  rid  of  possible  conduction 
currents  across  the  surface  of  the  insulator. 

An  apparatus  of  this  kind  is  very  suitable  for  determining 
the  decay  curves  of  the  excited  activity  imparted  to  cylindrical 
electrodes,  and  for  determinations  of  the  decay  of  activity  of 
emanations  which  are  introduced  into  the  cylinder. 

The  electric  method  is  an  extraordinarily  delicate  means  of 
detecting  the  presence  of  minute  quantities  of  radioactive  matter. 
This  can  be  readily  illustrated  by  a  simple  experiment.  A 
milligram  of  radium  bromide  is  taken  and  dissolved  in  100  c.c. 
of  water.  After  thorough  mixing,  1  c.c.  of  this  solution  is 
taken  and  added  to  99  c.c.  of  water.  One  c.c.  of  this  last 
solution  thus  contains  10~7  gram  of  radium  bromide.  If  this 
is  evaporated  on  a  metal  vessel,  the  activity  possessed  by  this 
minute  quantity  of  radium  suffices  to  cause  an  extremely  rapid 
discharge  when  brought  near  the  cap  of  an  electroscope,  such 
as  is  shown  in  Fig.  11.  If  the  radium  covered  plate  is  placed  on 
the  cap  of  the  electroscope,  it  is  impossible  for  the  leaves  to 
retain  their  charge  more  than  a  few  seconds. 

Using  an  electroscope  of  small  natural  leak,  the  presence  of 
10~n  gram  of  radium  can  easily  be  detected  by  the  increased 
rate  of  movement  imparted  to  the  gold  leaf. 

Extraordinarily  minute  currents  can  be  measured  with  ac- 
curacy in  an  electroscope  of  the  type  shown  in  Fig.  6.  For 
example,  Cooke  observed  that  in  a  well  cleaned  brass  vessel  of 
about  one  litre  capacity,  the  fall  of  potential  due  to  the  natural 
ionization  of  the  air  inside  the  electroscope  was  about  six  volts 
per  hour.  The  capacity  of  the  gold-leaf  system  was  about  one 
electrostatic  unit.  The  current  is  equal  to  the  capacity  multi- 
plied by  the  fall  of  potential  per  second,  i.  e., 


1x6 

current  =       — ^-^  =  5.6  x  10~6  electrostatic  units 

X  oUO 

=  1.9  X  10~15  amperes. 


36  RADIOACTIVE   TRANSFORMATIONS 

With  special  precautions,  a  rate   of  discharge  of   -^  of  this 
amount  can  be  accurately  measured. 

The  number  of  ions  produced  in  the  electroscope  can  readily 
be  deduced.  J.  J.  Thomson  has  shown  that  an  ion  carries 
a  charge  of  3.4  x  1Q-10  electrostatic  units  or  1.13  x  10~19 
coulombs.  The  number  of  ions  produced  per  second  in  the 
air  is  thus, 

1.9  x  10-15 


1.13  x  10~19 


=  17000. 


Supposing  the  ionization  to  be  uniform  throughout  the  volume 
of  air,  this  corresponds  to  a  production  of  17  ions  per  c.c. 
per  second  in  the  volume  of  air  inside  the  vessel  of  one  litre 
capacity. 

It  will  be  shown  later  that  the  average  a  particle  is  able  to 
produce  about  one  hundred  thousand  ions  in  its  path  before  it 
ceases  to  ionize  the  gas.  We  thus  see  that  the  electrical  method 
is  capable  of  detecting  the  ionization  produced  by  a  quantity  of 
active  matter  which  on  an  average  expels  one  a  particle  per 
second ;  or,  in  other  words,  the  electric  method  serves  to  detect 
a  transformation  of  matter  which  takes  place  at  the  rate  of  one 

atom  per  second. 

^  **        \  •  t 
For  the  detection  of  matter  which  possesses  the  radioactive* 

property,  the  electric  method  thus  far  transcends  in  delicacy  the 
use  of  the  spectroscope.  In  consequence  of  this,  we  are  able  to 
detect  the  presence  of  an  active  substance  like  radium,  when 
it  exists  in  almost  infinitesimal  amount  mixed  with  inactive 
matter,  and  also  to  determine  with  fair  accuracy  the  amount 
present.  This  test  is  so  extraordinarily  delicate  that  the  pres- 
ence of  minute  traces  of  radium  has  been  observed  in  almost 
every  substance  which  has  been  examined. 


CHAPTER  II 
RADIOACTIVE  CHANGES  IN  THORIUM 

IN  the  preceding  chapter,  a  brief  survey  has  been  made  of  the 
more  important  properties  possessed  by  the  radioactive  bodies, 
and  a  short  description  has  been  given  of  the  various  methods 
of  measurement  of  radioactive  quantities.  In  this  arid  succeed- 
ing chapters,  we  shall  analyze  in  more  detail  the  processes  oc- 
curring in  the  radiaoctive  substances  and  the  theories  advanced 
for  their  explanation.  Although  in  most  of  the  subsequent 
chapters  we  shall  discuss  the  properties  of  radium  as  our  typical 
radioelement,  yet  the  changes  which  occur  in  radium  are  so 
numerous  and  so  complicated  that  it  is  advisable  to  consider  a 
simpler  illustration  before  entering  upon  the  more  complex 
problem. 

For  this  purpose,  we  shall  first  consider  the  succession  of 
changes  taking  place  in  thorium ;  for  in  that  substance  the  analy- 
sis required  is  of  a  much  simpler  character  than  for  radium 
itself.  In  this  way  we  shall  be  able  to  concentrate  our  atten- 
tion upon  the  main  phenomena  under  consideration  without 
being  too  much  disturbed  by  details.  When  once  the  general 
principles  involved  have  been  made  clear,  their  application  to 
the  more  complex  problem  of  radium  will  not  present  much 
Additional  difficulty. 

This  mode  of  beginning  is  also  historically  interesting,  for  it 
was  as  a  result  of  the  examination  of  the  processes  occurring  in 
thorium  that  the  disintegration  theory  of  radioactivity,  which 
will  form  the  basis  of  the  explanation  of  radioactive  phenomena, 
was  first  outlined. 

Thorium  compounds  have  about  the  same  radioactivity, 
weight  for  weight,  as  the  corresponding  compounds  of  uranium, 
and,  like  that  substance,  emit  a,  /5,  and  7  rays.  We  have  seen, 
however,  that  thorium  differs  from  uranium  in  emitting,  besides 


38  RADIOACTIVE   TRANSFORMATIONS 

the  three  types  of  rays,  an  "emanation,"  or  radioactive  gas. 
This  property  possessed  by  thorium  of  emitting  a  volatile  radio- 
active substance,  can  be  readily  illustrated  by  the  simple  experi- 
mental arrangement  shown  in  Fig.  11. 

A  glass  tube,  A,  is  filled  with  about  50  grams  of  thorium 
oxide,  or,  still  better,  thorium  hydroxide,  which  emits  the 
emanation  more  freely  than  the  oxide.  This  is  connected  by 
a.  narrow  tube,  L,  of  about  a  metre  length,  with  a  suitable,  well 
insulated  electroscope.  On  charging  the  electroscope,  the  leaves 
converge  very  slowly  as  the  thorium  compound  has  no  appreci- 
able effect  in  ionizing  the  gas  inside  the  electroscope.  If,  how- 
ever, a  slow  steady  stream  of  air  from  a  gas  bag  or  gasometer 


7 

+/ 


FIG.  11. 
Discharging  power  of  the  thorium  emanation. 

is  passed  over  the  thorium,  no  effect  is  observed  in  the  electro- 
scope for  a  definite  interval,  which  is  a  measure  of  the  time 
taken  for  the  air  current  to  travel  from  A  to  the  electroscope. 
The  leaves  are  then  seen  to  collapse  rapidly,  the  rate  of  move- 
ment increasing  for  several  minutes.  This  discharge  of  the 
electroscope  is  due  to  the  ionizing  effect  of  the  thorium  emana- 
tion, which  has  been  conveyed  with  the  current  of  air  into  the 
electroscope.  On  stopping  the  air  current,  the  rate  of  collapse 
of  the  gold  leaves  is  seen  to  diminish  steadily,  falling  to  half 
of  its  value  in  about  one  minute,  or,  to  be  more  accurate,  in  54 
seconds.  After  about  5  minutes  the  effect  of  the  emanation  is 
almost  inappreciable.  The  emanation,  left  to  itself,  loses  its 
activity  according  to  an  exponential  law.  In  the  first  54  seconds 
the  activity  is  reduced  to  half  value ;  in  twice  that  time,  i.  e.  in 
108  seconds,  the  activity  is  reduced  to  one  quarter  value,  and 


RADIOACTIVE   CHANGES   IN   THORIUM         39 

in  162  seconds  to  one  eighth  value,  and  so  on.  This  rate  of 
decay  of  the  activity  of  the  thorium  emanation  is  its  charac- 
teristic feature,  and  serves  as  a  definite  physical  method  of  dis- 
tinguishing the  thorium  emanation  from  that  of  radium  or  of 
actinium  which  decay  at  very  different  rates. 

Although  the  amount  of  emanation  liberated  from  a  kilogram 
of  a  thorium  compound  is  far  too  minute  to  be  detected  either 
by  its  volume  or  weight,  yet  the  electrical  test  of  its  presence 
is  so  extraordinarily  delicate  that  its  discharging  effect  can 
readily  be  observed  with  only  a  few  milligrams  of  material. 

The  amount  of  emanation  liberated  into  the  air  from  a  given 
weight  of  thorium  varies  enormously  with  the  different  com- 
pounds. For  example,  it  is  emitted  freely  by  thorium  hydroxide, 
but  very  slightly  by  thorium  nitrate.  Rutherford  and  Soddy  1 
made  a  detailed  examination  of  the  "  emanating  power "  of 
thorium  compounds,  i.  e.  the  amount  of  emanation  given  off 
into  the  air  per  second  by  a  given  weight  of  material,  and  found 
that  it  was  much  affected  by  physical  and  chemical  conditions. 
It  is  increased  by  the  presence  of  moisture  and  by  a  rise  of 
temperature  up  to  a  red  heat.  Lowering  of  the  temperature  to 
—80°  C  diminishes  the  emanating  power  considerably  in  many 
cases.  The  emanation  is,  however,  given  off  freely,  and  to  an 
equal  extent  by  all  compounds  of  thorium  in  solution.  This  is 
most  readily  shown  by  bubbling  a  current  of  air  through  the 
solution,  when  part  of  the  emanation  escapes  mixed  with  the 
air.  Rutherford  and  Soddy  showed  that  the  great  variations 
in  emanating  power  under  different  conditions  were  not  due  to 
differences  in  the  rate  of  production  of  the  emanation  in  the 
various  cases,  but  merely  to  variations  in  the  rate  of  escape  of  the 
emanation  into  the  air.  Since  the  thorium  emanation  loses  a 
large  proportion  of  its  activity  in  a  few  minutes,  any  retarda- 
tion in  its  rate  of  escape  through  the  pores  of  a  solid  compound 
will  very  materially  alter  the  emanating  power.  They  con- 
cluded that,  for  equal  weights  of  the  element  thorium,  all 
thorium  compounds  produce  an  equal  quantity  of  the  emana- 

i  Rutherford  and  Soddy :  Phil.  Mag.,  Sept.,  1902. 


40  RADIOACTIVE   TRANSFORMATIONS 

tion  per  second,  but  that  the  rate  of  its  escape  into  the  gas 
depends  greatly  on  physical  and  chemical  conditions. 

We  shall  now  briefly  consider  the  chemical  nature  of  the  ema- 
nation itself,  disregarding  for  the  moment  the  substance  from 
which  it  arises.  Since  the  emanation  is  released  in  insignifi- 
cant amount,  no  direct  chemical  examination  can  be  made,  but 
the  conductivity  produced  in  a  gas  by  the  emanation  offers  a 
very  simple  method  of  testing  whether  its  amount  is  reduced 
after  it  has  been  acted  on  by  various  agents.  For  example,  if 
the  conductivity  produced  in  a  testing  vessel  is  unaltered  after 
passing  the  emanation  slowly  through  a  platinum  tube  at  a 
white  heat,  and  this  is  what  has  actually  been  observed,  we  can 
safely  conclude  that  the  emanation  is  unaffected  by  exposure  to 
that  temperature.  In  this  way  Rutherford  and  Soddy  found 
that  the  thorium  emanation  was  not  acted  on  appreciably  by 
any  physical  or  chemical  agent.  The  emanation  was  exposed 
to  such  severe  treatment  that  no  gas  except  one  of  the  inert 
group  of  the  argon-helium  family  could  have  possibly  survived 
the  various  processes  without  change  in  amount.  It  was  there- 
fore concluded  that  the  emanation  is  a  chemically  inert  gas, 
which,  in  respect  to  the  absence  of  any  definite  combining  prop- 
erties, is  chemically  allied  to  the  argon-helium  family. 

The  material  nature  of  the  emanation  was  strongly  confirmed 
by  the  fact  that  it  could  be  condensed  from  any  gas  with  which 
it  is  mixed,  by  the  action  of  extreme  cold.  The  thorium  emana- 
tion commences  to  condense  from  the'  air  at  a  temperature  of 
about  —120°  C.  The  emanation  is  in  consequence  completely 
stopped  by  passing  the  gas  with  which  it  is  mixed,  slowly 
through  a  U  tube  immersed  in  liquid  air.  From  the  rate  of 
diffusion  of  the  emanation  through  air  and  other  gases,  it  has 
been  deduced  that  the  emanation  is  a  gas  of  heavy  molecular 
weight. 

We  have  already  seen  that  the  emanation  loses  its  activity 
rapidly  according  to  an  exponential  law.  If  /0  is  the  initial 
activity,  the  activity  It  at  any  time  later  is  given  by  the 
equation 


RADIOACTIVE   CHANGES   IN   THORIUM 


41 


where  X  is  a  constant  and  e  is  the  base  of  Naperian  logarithms. 
Since  the  activity  falls  to  half  value  in  about  54  seconds  the 
value  of 

A  =  logg2  =  .0128  (sec)-1. 
54 

The  decay  curve  of  the  thorium  emanation  is  shown  in  Fig. 
12.     If  the  logarithm  of  the  activity  at  any  time  is  plotted  with 


/oo 


60 


/SO  tOO 

T/MC    IN  SCCQf*Q9. 


200 


2*0 


380* 


FIG.  12. 
Decay  of  activity  of  the  thorium  emanation. 

respect  to  time,  the  points  all  lie  in  a  straight  line.  This  is 
shown  in  the  figure,  the  initial  value  of  the  logarithm  of  the 
activity  being  taken  as  100  for  convenience. 

This  value  of  X  is  a  characteristic  constant  of  the  emanation 
and  will  be  termed  the  "radioactive  constant."  As  far  as  obser- 
vation has  at  present  shown,  its  value  is  independent  of  phys- 
ical or  chemical  conditions.  For  example,  the  value  of  X  is 


42  RADIOACTIVE    TRANSFORMATIONS 

the  same  for  the  emanation  when  condensed  by  liquid  air  at  a 
temperature  of  —186°  C,  as  under  normal  conditions. 

All  simple  radioactive  products  lose  their  activity  according 
to  an  exponential  law,  and  it  is  convenient  to  employ  a  single 
term  to  denote  the  time  taken  by  any  simple  product  to  lose 
half  of  its  activity.  The  term  "period  "  of  a  product  will  be 
used  in  this  sense  to  avoid  circumlocution. 

We  must  now  consider  the  interpretation  to  be  placed  upon 
the  observed  law  of  decay  of  activity  of  the  emanation.  Is  the 
activity  of  the  emanation  merely  a  transient  superficial  property, 
or  is  it  directly  connected  with  some  essential  change  in  the 
emanation  itself?  Consider  for  a  moment  how  the  activity  is 
measured.  The  emanation  gives  out  only  a  rays,  which,  as  we 
have  seen,  are  heavy  positively  charged  particles  projected  with 
a  speed  of  about  twenty  thousand  miles  per  second.  The  ioniza- 
tion  observed  in  the  gas  is  due  to  the  collision  of  the  projected  a 
particles  with  the  gas  molecules  which  lie  in  their  path.  The 
number  of  ions  produced  by  each  a  particle  under  ordinary  con- 
ditions is  very  great,  probably  amounting  in  some  cases  to  about 
one  hundred  thousand.  The  activity  measured  by  the  electric 
method  is  thus  a  comparative  measure  of  the  number  of  a  par- 
ticles expelled  from  the  emanation  per  second. 

The  a  particles  are  apparently  derived  from  the  atoms  of  the 
emanation,  and  indeed  it  is  difficult  to  avoid  the  conclusion 
that  they  were  not  projected  initially  from  rest,  but  were  in 
a  state  of  rapid  motion  before  their  escape  from  the  atom.  It 
can  be  calculated  that  the  a  particle  would  have  to  move  freely 
between  two  points  differing  in  potential  by  about  five  million 
volts  in  order  to  acquire  its  enormous  velocity  of  projection. 

It  is  difficult  to  imagine  any  mechanism  either  within  or  out- 
side the  atom  capable  of  suddenly  setting  in  such  rapid  motion 
so  heavy  a  mass  as  that  of  the  a  particle.  We  are  almost  forced 
to  the  conclusion  that  the  a  particle  was  originally  in  rapid  mo- 
tion within  the  atom  and  for  some  reason  suddenly  escaped  from 
the  atomic  system  with  the  velocity  it  originally  possessed  in  its 
orbit.  We  may  suppose  that  the  expulsion  of  an  a  particle  is 
a  result  of  a  violent  explosion  within  the  atom.  The  residual 


RADIOACTIVE   CHANGES   IN   THORIUM         43 

atom  is  lighter  than  before,  and  it  is  to  be  expected  that  its 
physical  and  chemical  properties  will  be  different  from  those  of 
the  parent  atom.  This,  as  we  shall  see  later,  is  observed  in 
all  similar  cases.  As  a  result  of  the  expulsion  of  a  particles, 
the  emanation  of  thorium  is  changed  into  another  distinct  sub- 
stance which  behaves  like  a  solid  and  is  deposited  on  the  sur- 
face of  bodies.  This  product  of  the  decomposition,  or  rather 
disintegration,  of  the  emanation  will  be  discussed  later. 

If  each  atom  of  the  emanation  on  breaking  up  expels  one  a 
particle,  the  observed  law  of  decay  of  activity  is  expressed  by 
the  equation 


where  nQ  is  the  initial  number  that  breaks  up  per  second,  nt  is 
the  number  at  any  time  t  and  X  is  the  radioactive  constant. 
The  same  equation  also  holds  if  each  atom  in  disintegrating 
expels  two  or  more  a  particles. 

Assuming  for  the  sake  of  simplicity  the  probable  law  that 
the  disintegration  of  each  atom  is  accompanied  by  the  appear- 
ance of  one  a  particle,  the  number  NQ  of  atoms  of  emanation 
initially  present  must  be  equal  to  the  total  number  of  a  particles 
expelled  during  the  whole  life  of  the  emanation.  This  number 
is  given  by 

NQ  =    f  °°  nt  dt  =  nQ  f 

t/O  t/O 


The  number  TV  which  remain  unchanged  after  a  period  t  is  easily 
seen  to  be  given  by 


-£ 


Then 


We  have  thus  arrived  at  the  important  conclusion  that  the 
number  of  atoms  of  the  emanation  which  remain  unchanged 
at  any  time  decreases  in  exactly  the  same  way  as  the  activity, 
or,  in  other  words,  the  activity  of  the  emanation  is  directly 


44  RADIOACTIVE   TRANSFORMATIONS 

proportional  to  the  number  of  atoms  of  emanation  present. 
This  is  the  result  naturally  to  be  expected  from  a  priori  con- 
siderations. 

The  exponential  law  of  change  of  radioactive  matter  is  the 
same  as  for  a  so-called  monomolecular  change  in  chemistry, 
observed  in  special  cases  when  one  of  the  two  combining  sub- 
stances is  present  in  a  very  large  proportion  compared  with  the 
other.  The  fact  that  the  constant  of  decay  is  independent  of 
the  concentration  of  the  emanation  points  to  the  conclusion  that 
only  one  changing  system  is  involved.  The  fact  that  the  con- 
stant of  decay  does  not  depend  on  the  physical  and  chemical 
conditions  suggests  that  the  changing  system  is  the  atom  itself 
and  not  the  molecule. 

The  radioactive  constant  X  has  a  definite  physical  meaning. 
We  have  seen  above  that 


Differentiating  with  regard  to  the  time, 

dN 


or,  the  average  number  of  'atoms  which  break  up  per  second  is 
equal  to  the  total  number  present  multiplied  by  X.  The  value 
of  X  thus  represents  the  fraction  of  the  total  number  of  atoms 
which  disintegrate  per  second.  For  the  thorium  emanation  in 
which  half  of  the  emanation  breaks  up  in  54  seconds,  the  ratio 
X  =  .0128  (sec)"1.  For  example,  suppose  a  vessel  contained 
initiallv  10,000  atoms  of  the  thorium  emanation.  In  the  first 
second,  on  the  average,  128  atoms  break  up  per  second.  After  54 
seconds,  the  number  of  atoms  of  emanation  present  is  5000,  and 
the  number  breaking  up  per  second  is  64.  After  108  seconds 
the  number  of  atoms  of  emanation  left  is  2500,  and  32  bres 
up  per  second,  and  so  on.  The  value  of  X  has  thus  a  definil 
and  important  physical  meaning  for  any  radioactive  product. 

We  have  spent  some  time  in  considering  the  physical  intei 
pretation  to  be  placed  upon  the  observed  law  of  decay  of  activ- 
ity for  the  emanation,  since  every  radioactive  product,  so  fa 


RADIOACTIVE   CHANGES   IN   THORIUM         45 

examined,  follows  the  same  law  of  change,  but  with  a  different 
though  definite  value  of  X,  which  is  characteristic  for  each 
special  type  of  radioactive  matter.  The  same  general  explana- 
tion of  the  decay  of  activity  may  thus  be  directly  applied  to  any 
radioactive  product. 

EXCITED  RADIOACTIVITY  OF  THOBIUM 

The  writer1  showed  that  thorium  compounds,  besides  emit- 
ting three  types  of  rays  and  an  emanation,  possessed  the  follow- 


Thorium  Oxide 


Battery 

FIG.  13. 
Concentration  of  the  excited  activity  on  the  negative  electrode. 

ing  remarkable  property.     Any  body  which  has  been  exposed  1 
in  the  presence  of  the  thorium  emanation  becomes  itself  radio-  [ 
active.     This  "excited"  or  "induced"  radioactivity,  as  it  has 
been  termed,  is  not  permanent,  but  decays  when  the  body  is  re- 
moved from  the  presence  of  the  emanation.     The  activity  can 
be  largely  concentrated  on  the  negative  electrode  in  a  strong 
electric  field.     This  can  readily  be  shown  by  exposing  a  fine 
wire,  AB,  (Fig.  13)  in  the  presence  of  an  emanating  thorium 
compound  contained  in  a  closed  box,  V. 

When  the  wire  is  the  only  negatively  charged  body  exposed 
to  the  emanation,  it  becomes  intensely  active.  If  it  is  charged 
positively,  very  little  activity  is  observed.  A  fine  wire  may  in 

1  Rutherford  :  Phil.  Mag.,  Jan.  and  Feb.,  1900. 


46  RADIOACTIVE   TRANSFORMATIONS 

this  way  be  made  many  hundred  times  more  active  per  unit  area 
than  thorium  itself.  The  activity  is  due  to  a  material  deposit 
on  the  wire,  for  it  can  be  partly  removed  by  rubbing  the  wire 
with  a  piece  of  cloth,  and  can  be  dissolved  off  a  platinum  wire 
by  strong  acids.  If  the  acid  is  evaporated  in  a  dish  to  dryness, 
the  active  residue  is  left  behind.  The  active  matter  can  also 
be  driven  off  by  exposing  the  platinum  wire  to  a  temperature 
above  a  red  heat.  This  property  of  the  "active  deposit,"  as  it 
will  be  called,  will  be  discussed  in  more  detail  later.  The 
amount  of  activity  under  given  conditions  which  can  be  con- 
centrated on  a  body  is  independent  of  its  chemical  nature. 
Every  substance  made  active  in  this  way  behaves  as  if  it  were 
coated  with  an  invisible  film  of  the  same  radioactive  material. 
Although  the  active  deposit  is  too  small  in  quantity  to  be 
directly  observed,  the  electrical  effects  produced  by  it  are  often 
large  and  very  readily  measured. 

CONNECTION  BETWEEN  THE  ACTIVE  DEPOSIT  AND  THE 
EMANATION 

The  property  of  producing  an  active  deposit  on  bodies  belongs 
not  to  thorium  directly,  but  to  the  emanation  emitted  by  it. 
The  activity  of  the  deposit,  produced  by  exposure  of  a  body 
near  a  thorium  compound,  depends  on  the  emanating  power  of 
the  compound.  It  is  much  greater,  for  example,  for  the  hy- 
droxide than  for  the  nitrate,  since  the  former  emits  emanation 
much  more  freely.  If  the  thorium  compound  is  completely 
covered  by  a  very  thin  plate  of  mica  which  prevents  the  escape 
of  the  emanation,  no  excited  activity  is  produced  on  a  body 
placed  outside  it.  This  shows  that  the  excited  activity  is  not 
a  consequence  of  some  action  of  the  rays  directly  emitted  from 
thorium,  since  these  are  only  slightly  stopped  by  the  mica,  but 
is  due  to  the  presence  of  the  emanation.  The  close  connection 
between  the  emanation  and  excited  activity  is  clearly  brought 
out  by  the  following  experiment.  A  slow  constant  stream  of 
air  is  passed  over  a  large  weight  of  thorium  compound,  and  the 
stream  of  air  mixed  with  emanation  is  then  passed  through 
a  long  tube,  in  which  cylindrical  electrodes,  A,  B,  C,  of  equal 


RADIOACTIVE   CHANGES   IN   ^HORIUM         47 

length,  are  placed.  The  arrangement  is  clearly  seen  in  Fig.  14. 
The  conductivity  of  the  gas,  which  is  a  measure  of  the  amount 
of  emanation  present,  falls  off  from  electrode  to  electrode,  since 
the  emanation  loses  its  activity  with  time.  The  ionization  cur- 
rent observed,  for  example,  between  the  electrode  A  and  the 
outer  cylinder  is  at  first  a  measure  of  the  amount  of  emanation 
present  in  the  space  between  the  cylinder  and  the  electrode. 
After  several  hours,  the  stream  of  air  was  stopped,  the  central 
rods,  A,  B,  C,  were  removed,  and  the  excited  activity  deter- 
mined by  the  electric  method  in  an  apparatus  similar  to  that 
shown  in  Fig.  9.  The  excited  activity  was  observed  to  fall  off 
from  electrode  to  electrode  in  exactly  the  same  proportion  as  the 


FIG.  14. 

An  experiment  to  show  the  connection  between  the  emanation  and  the  excited 
activit}'  it  produces. 

activity  due  to  the  emanation  alone.  This  shows  that  the  ex- 
cited activity  produced  is  directly  proportional  to  the  amount 
of  emanation  present;  for  as  the  amount  of  emanation  decreases, 
the  excited  activity  falls  off  in  the  same  ratio.  This  experiment 
also  shows  conclusively  that  the  emanation  causes  the  excited 
activity,  since  the  latter  is  produced  at  points  far  removed  from 
the  action  of  the  direct  radiation  from  the  thorium.  The  pro- 
portion that  exists  between  the  amount  of  the  active  deposit 
and  of  the  emanation  shows  clearly  that  the  latter  is  the  parent 
of  the  former.  It  may  be  supposed  that  the  residue  of  the  atom 
of  the  emanation  after  an  expulsion  of  the  a  particle  becomes 
the  atom  of  the  active  deposit.  The  new  atom  in  some  way 
gains  a  positive  charge,  and  is  conveyed  to  the  negative  elec- 
trode to  which  it  adheres.  In  the  absence  of  an  electric  field, 


48  RADIOACTIVE   TRANSFORMATIONS 

the  carriers  of  the  active  substance  are  conveyed  by  diffusion  to 
the  sides  of  the  vessel  containing  the  emanation.  The  number 
of  particles  of  the  active  deposit  produced  per  second  in  any 
space  should  be  proportional  to  the  number  of  particles  of 
emanation  which  break  up  per  second.  The  latter  number,  as 
we  have  seen,  is  proportional  to  the  activity  measured  by  the 
electrometer.  The  view  that  the  particles  of  the  active  deposit 
result  from  the  disintegration  of  the  emanation  thus  leads  at 
once  to  the  conclusion  that  the  amount  of  excited  activity  pro- 
duced in  any  space  is  directly  proportional  to  the  activity  of 
the  emanation  present,  i.  e.  to  the  amount  of  emanation  present. 
We  may  thus  conclude  with  confidence  that  the  active  deposit 
is  derived  from  the  disintegration  of  the  emanation,  or,  in  other 
words,  that  the  emanation  is  the  parent  of  the  active  deposit. 

There  is  a  striking  difference  between  the  physical  and 
chemical  properties  of  the  active  deposit  and  of  its  parent  the 
emanation.  The  latter,  as  we  have  pointed  out,  is  a  chemically 
inert  gas,  insoluble  in  acids,  which  condenses  at  about  —120°  C. 
The  former  behaves  as  a  solid  substance,  is  soluble  in  strong 
acids,  and  volatilizes  at  a  temperature  above  a  red  heat.  As  a 
result  of  the  disintegration  of  the  emanation,  a  new  substance 
is  produced  which  differs  completely  both  in  physical  and 
chemical  properties  from  its  parent. 

COMPLEXITY  OF  THE  ACTIVE  DEPOSIT 

If  a  body  is  exposed  for  several  days  in  the  presence  of  the 
emanation,  the  excited  activity  after  removal  decays  very  nearly 
.according  to  an  exponential  law,  falling  to  half  value  in  11 
hours.  The  active  deposit  emits  a,  {3,  and  7  rays,  and  the  rate 
of  decay  is  the  same  whichever  type  of  rays  is  used  as  a  means 
of  measurement.  This  at  once  suggests  that  a  type  of  radio- 
active matter  may  be  present  which  is  half  transformed  in  about 
11  hours,  emitting  during  the  process  the  three  types  of  rays. 
The  active  deposit  is,  however,  more  complex  than  this  would 
indicate.  If  a  body  is  exposed  for  only  a  few  minutes  to  a 
large  supply  of  emanation,  the  activity  after  removal  varies  in 
.a  very  different  way  from  that  observed  after  a  long  exposure 


RADIOACTIVE   CHANGES   IN  THORIUM 


49 


to  the  emanation.  The  activity  is  very  small  at  first,  but 
steadily  increases  for  about  3.66  hours,  when  it  reaches  a  maxi- 
mum value.  After  six  hours,  the  activity  diminishes  accord- 
ing to  the  eleven  hour  period,  observed  in  the  case  of  long 
exposures.  The  curve  of  decay  is  shown  in  Fig.  15. 

This  variation  of  the  activity  appears  at  first  sight  very  re- 
markable and  difficult  to  explain.     The  curve  of  variation  of 


IZO 


10  Q 


60 


20 


•40 


/20 


/oo  20  o 

TV/VfC  //V  f*/MVTCS. 


320 


FIG.  15. 

Variation  with  time  of  the  excited  activity  produced  after  a  short  exposure 
to  the  thorium  emanation. 


activity  is  quite  independent  of  the  chemical  nature  of  the  body 
on  which  the  active  deposit  is  obtained.  It  is  shown  equally 
for  deposits  on  thick  plates  of  metal,  and  on  the  thinnest  sheets 
of  metal  foil.  The  curve,  however,  can  be  completely  explained 
if  the  active  deposit  contains  two  distinct  substances,  one  of 
which  is  produced  from  the  other.  Let  us  suppose,  for  the  mo- 
ment, that  the  emanation  changes  into  a  substance  called  thorium 
A,  and  that  after  deposit  on  the  surface  of  the  body,  thorium 


50  RADIOACTIVE   TRANSFORMATIONS 

A  is  gradually  transformed  into  another  distinct  substance, 
thorium  B.  The  substance  thorium  A  is  supposed  to  be  trans- 
formed without  the  emission  of  either  a,  /3,  or  7  rays,  but  thorium 
B  emits  all  three  types  of  rays.  If  the  time  of  exposure  to  the 
emanation  is  very  short  compared  with  the  period  of  transforma- 
tion of  thorium  A  or  B,  the  matter  deposited  from  the  emana- 
tion will  at  first  consist  almost  entirely  of  the  inactive  substance 
thorium  A.  The  initial  activity  after  removal  will  in  conse- 
quence be  very  small.  Since  A  gradually  changes  into  B,  and 
since  the  transformation  of  B  is  accompanied  by  the  emission  of 
rays,  the  activity  will  at  first  increase  with  the  time,  for  more 
and  more  of  B  is  produced.  After  a  definite  interval,  the  rate 
of  transformation  of  B  will  exactly  compensate  for  the  rate  of 
supply  of  B  due  to  the  change  in  A.  At  that  moment  the  ac- 
tivity will  be  a  maximum,  and  will  afterwards  decrease,  since 
the  amount  of  B  will  steadily  diminish.  This  hypothesis  is 
seen  to  account  in  a  general  way  for  the  shape  of  the  curve,  but 
we  shall  now  show  that  it  also  offers  a  complete  quantitative  ex- 
planation. Suppose  that  the  constants  of  change  of  A  and  B 
are  Xj  and  X2  respectively,  and  that  n^  atoms  of  A  are  deposited 
on  the  body  during  its  short  exposure  to  the  emanation.  After 
removal  of  the  body,  the  number  P  of  atoms  of  A  diminishes 
according  to  the  equation 


P  = 
The  rate  of  change  of  P  is  given  by 


If  Q  is  the  number  of  atoms  of  B  present  at  any  time  t  later,  the 
rate  of  increase  of  Q  is  equal  to  the  rate  of  supply  of  fresh 
atoms  in  consequence  of  the  transformation  of  A,  less  the  rate 
of  change  of  the  atoms  of  B  itself,  i.  e., 


(1) 


RADIOACTIVE   CHANGES    IN   THORIUM        51 

The  solution  of  this  equation  is  seen  to  be  of  the  form 

Q  =  a  e-V  +  b  e-W  ; 
and  since  Q  —  0  when  t  =  0, 

a  +  b  =  0. 
By  substitution  we  find  that 


Therefore, 


. 

A2  —  A! 

The  value  of  Q  increases  at  first  with  the  time,  passes  through 
a  maximum,  and  then  diminishes.  A  maximum  is  obtained 
when 


i.  e.,  at  a  time  T  given  by  the  equation 


Since  B  alone  gives  out  rays,  the  activity  It  at  any  time  t  is 
always  proportional  to  the  amount  of  B  present:  i.  e.,  to  the 
value  of  .  We  therefore  see  that 


_*  —    v          "     '  (2\ 

^f^  —         \  T  A  T>  \    y 

t  st A*  J.  rt—  At  ±    7  .  \      / 


where  li  is  the  maximum  activity.  Since  the  activity,  whether 
for  a  long  or  short  exposure,  finally  decays  according  to  an  ex- 
ponential law  with  the  period  of  11  hours,  it  follows,  that  either 
thorium  A  or  thorium  B  is  transformed  according  to  this  period. 
Let  us  suppose  for  the  moment  that  half  of  A  is  transformed  in 
11  hours.  The  corresponding  value  of  \  is  1.75  x  10~5(sec)~1. 
It  now  remains  for  us  to  determine  the  period  of  B  from  con- 
sideration of  the  experimental  curve.  Since  it  is  observed  that 


52 


RADIOACTIVE   TRANSFORMATIONS 


the  activity  reaches  a  maximum  value  after  a  time  T  =  220 
minutes,  on  substituting  the  values  of  Xj  and  T  in  equation 
(2),  the  value  of  X2  is  found  to  be 

A2  =  2.08  X  10~4  (sec)-1. 

This  corresponds  to  a  change  in  which  half  of  the  matter  of 
thorium  B  is  transformed  in  55  minutes.    On  substituting  these 

values  of  Xj,  X2,  and  T  in  equation  (2),  the  value    of  ~  can 

1T 

be  at  once  determined.  The  very  close  agreement  between 
the  values  deduced  from  the  theory  and  the  experimental 
numbers  is  shown  in  the  following  table. 


Time  in  minutes. 

It 
Theoretical  values  of  jf 

lf 

Observed  values  of  -f-. 
1T 

15 

.22 

.23 

30 

.38 

.37 

60 

.64 

.63 

120 

.90 

.91 

220 

1.00 

1.00 

305     , 

.97 

.96 

The  agreement  is  equally  close  for  still  longer  periods.  After 
about  6  hours  the  activity  decreases  very  nearly  exponentially, 
falling  to  about  half  value  in  11  hours. 

We  thus  see  that  a  quantitative  explanation  of  the  activity 
curve  can  be  obtained  on  the  following  assumptions :  — 

(1)  That  the  matter  thorium  A  deposited  from  the  emanation 
is  half  transformed  in  11  hours,  but  does  not  itself  emit  rays. 

(2)  That  the  matter   thorium  A  changes  into  thorium  B. 
which  is  half  transformed  in  55  minutes,  and  emits  all  three 
types  of  rays. 

A  very  interesting  point  arises  in  the  selection  of  the  periods 
of  the  transformation  of  thorium  A  and  B.  We  assumed  that 
the  period  of  11  hours  belonged  to  A  rather  than  to  B,  but  the 
activity  curve  itself  gives  us  no  information  on  this  question. 
We  see  that  equation  (2)  is  symmetrical  in  regard  to  Xj,  X2, 


RADIOACTIVE   CHANGES   IN   THORIUM         5B 

and  in  consequence  would  not  be  altered  by  an  interchange  of 
their  values.  In  order  to  settle  this  question  definitely,  it  is 
necessary  to  isolate  thorium  B  from  the  mixture  of  A  and  B, 
and  separately  determine  its  period.  If  it  is  found  possible  to 
isolate  an  active  product  from  the  mixture  of  A  and  B  which 
decays  exponentially,  falling  off  to  half  value  in  55  minutes, 
it  follows  at  once  that  the  "ray  "  product  B  has  this  period,  and 
that  the  11  hour  period  belongs  to  A,  the  "rayless"  product. 
This  separation  has  actually  been  accomplished  by  several  ex- 
perimental methods,  and  the  results  completely  confirm  the 
theory  already  considered,  and  at  the  same  time  illustrate  in  a 
remarkable  way  the  differences  in  physical  and  chemical  proper- 
ties of  the  two  products,  thorium  A  and  B. 

Pegram  l  examined  the  radioactivity  produced  in  the  elec- 
trodes by  electrolysis  of  a  thorium  solution,  and,  under  suitable 
conditions,  obtained  a  product,  the  activity  of  which  decayed 
exponentially,  falling  to  half  value  in  about  1  hour. 

Von  Lerch 2  made  a  number  of  experiments  on  the  effect  of 
electrolyzing  a  solution  of  the  active  deposit  of  thorium,  ob- 
tained by  solution  in  hydrochloric  acid  of  the  active  deposit  on 
a  platinum  wire.  Deposits  of  varying  rates  of  decay  were  ob- 
tained under  different  conditions,  some  decaying  to  half  value 
in  11  hours,  and  others  at  a  more  rapid  rate.  By  using  nickel 
electrodes,  he  obtained  an  active  substance  which  decayed  ex- 
ponentially, falling  to  half  value  in  one  hour.  Considering  the 
close  agreement  between  the  calculated  and  observed  periods, 
viz.,  55  and  60  minutes  respectively,  there  can  be  no  doubt  that 
the  ray  product,  thorium  B,  had  been  completely  separated 
from  the  mixture  of  A  and  B  by  electrolysis.  The  rates  of 
decay  of  the  deposits  obtained  under  different  conditions  are 
readily  explained,  for  in  most  cases  A  and  B  are  deposited  elec- 
trolytically  together,  but  in  varying  proportions. 

This  result  was  still  further  confirmed  by  Miss  Slater,3  using 
a  different  method.  A  platinum  wire,  made  active  by  exposure 

1  Pegram:  Phys.  Rev.,  Dec.,  1903. 

2  Von  Lerch:  Annal.  d.  Phys.,  Nov.,  1903;  Akad.  Wiss.  Wien,  March,  1905. 

3  Miss  Slater :  Phil  Mag.,  May,  1905. 


54  RADIOACTIVE   TRANSFORMATIONS 

to  the  emanation,  was  heated  to  a  high  temperature  by  means 
of  an  electric  current.  Miss  Gates  had  previously  l  observed 
that  although  the  activity  of  a  platinum  wire  was  lost  by  heat- 
ing to  a  white  heat,  yet  if  the  heated  body  was  surrounded  by 
a  cold  tube,  the  activity  after  heating  was  found  to  be  dis- 
tributed in  undiminished  amount  over  the  interior  of  the  tube. 
This  experiment  showed  that  the  activity  had  not  been  destroyed 
by  the  action  of  a  high  temperature,  but  that  the  active  matter 
had  been  volatilized  by  heat  and  redeposited  on  the  surrounding 
cold  bodies.  Miss  Slater  examined  the  rates  of  decay,  both  of 
the  activity  left  behind  on  the  wire,  and  of  that  distributed  on 
a  lead  cylinder  surrounding  the  wire,  after  heating  the  latter 
for  a  short  time  at  different  temperatures.  After  exposure  to 
a  temperature  of  700°  C.  for  a  few  minutes,  the  activity  of  the 
wire  was  slightly  reduced.  The  activity  on  the  lead  cylinder 
was  small  at  first,  but  increased,  reaching  a  maximum  after 
about  4  hours,  and  then  decaying  exponentially  with  a  period  of 
11  hours.  This  variation  of  activity  is  almost  exactly  the  same 
as  that  observed  (Fig.  15)  for  a  wire  exposed  for  a  short  in- 
terval to  the  thorium  emanation;  i.  e.,  under  conditions  in 
which  the  matter  initially  consisted  almost  entirely  of  thorium  A. 
This  result,  then,  shows  that  some  thorium  A  was  driven  off  by 
heat,  and  deposited  on  the  surface  of  the  lead  cylinder.  On 
heating  to  about  1000°  C.  nearly  all  the  thorium  A  was  re- 
moved, for  it  was  observed  that  the  activity  left  behind  on  the 
wire  decayed  exponentially,  falling  to  half  value  in  about  1 
hour.  At  a  temperature  of  1200°  C.,  nearly  all  the  thorium  B 
also  was  volatilized.  These  results  thus  show  conclusively  that 
the  period  of  the  ray  product,  thorium  B,  is  about  1  hour,  and 
that  the  period  of  11  hours  must  be  ascribed  to  the  rayless 
product,  thorium  A.  We  therefore  see  that  it  has  been  found 
possible  to  isolate  the  components  of  a  mixture  of  thorium  A 
and  B  by  two  distinct  methods,  the  one  depending  on  the  dif- 
ference in  electrolytic  behavior  of  the  two  substances,  and  the 
other  on  their  difference  in  volatility.  This  is  a  very  interesting 
result,  for  it  not  only  indicates  the  difference  in  physical  and 

1  Miss  Gates:  Phys.  "Rev.,  p.  300,  1903. 


RADIOACTIVE   CHANGES   IN   THORIUM         55 

chemical  nature  of  the  two  components  of  the  active  deposit,  but 
also  shows  how  a  separation  of  two  substances  existing  together 
in  almost  infinitesimal  amounts  can  be  effected  by  specially 
devised  methods. 

It  is  at  first  sight  a  most  surprising  result  that  we  are 
able,  not  only  to  detect  the  presence,  but  also  to  determine  the 
physical  and  chemical  properties  of  a  product  like  thorium  A, 
which  does  not  manifest  its  presence  by  the  emission  of  radia- 
tions. This,  as  we  have  seen,  is  rendered  possible  by  the  fact 
that  the  product  of  its  transformation  emits  rays.  But  for  this 
property  the  presence  of  thorium  A  or  B  would  never  have  been 
detected  by  the  means  at  our  disposal. 

It  has  been  seen  that  after  a  long  exposure  to  the  thorium 
emanation,  the  excited  activity  at  once  commences  to  diminish. 
This  result  necessarily  follows  from  general  considerations.  If 
the  body  is  exposed  in  the  presence  of  a  constant  supply  of  the 
emanation  for  about  a  week,  the  activity  produced  reaches  a 
steady  limiting  value.  When  this  is  the  case,  the  number  of 
atoms  of  each  product  supplied  per  second  is  equal  to  the 
number  of  each  which  breaks  up  per  second.  Immediately 
after  removal  from  the  emanation,  the  amount  of  A  begins 
to  diminish  according  to  an  exponential  law,  and  it  can  be 
shown  both  theoretically  and  experimentally  that  the  activity, 
which  is  a  measure  of  the  amount  of  thorium  B,  does  not  at 
first  diminish  accurately  according  to  an  exponential  law,  with 
an  11  hour  period,  but  somewhat  more  slowly.  Several  hours 
after  removal,  however,  the  decay  is  very  nearly  exponential. 

It  is  a  matter  of  interest  to  observe  that  the  activity  for  a 
long  exposure  does  not  decay  according  to  the  period  of  the 
ray-emitting  substance,  but  according  to  that  of  the  "rayless" 
product.  The  decay  in  such  a  case  will  always  follow  the 
longer  period,  no  matter  whether  the  substance,  which  is  trans- 
formed according  to  that  period,  gives  out  rays  or  not. 

We  may  at  this  stage  briefly  summarize  the  conclusions 
arrived  at: 

"(1)  The  thorium  emanation  is  a  gas  which  is  half  transformed 
in  54  seconds,  and  emits  only  a  rays. 


56  RADIOACTIVE   TRANSFORMATIONS 

(2)  The  emanation  changes  into  a  solid  called  thorium  A, 
which  is  half  transformed  in  11  hours,  but  which  does  not  emit 
rays. 

(3)  The  thorium  A  in  turn  changes  into  a  product,  thorium 
B,  emitting  a,  /3,  and  7  rays,  which  is  half  transformed  in  about 
1  hour. 

The  successive  changes  occurring  in  the  emanation  are  shown 
diagrammatically  below : 

a  particle  a  particle 

A  A 


ray 
Emanation  — >  Thorium  A  — >  Thorium  B  — >  ? 

At  present  we  have  no  definite  information  with  regard  to  the 
product  of  transformation  of  thorium  B.  It  is  either  inactive, 
or  active  to  such  a  minute  extent  that  its  properties  cannot  be 
determined  by  the  electric  method. 

^N 

SEPARATION  OF  THORIUM  X 

It  is  now  necessary  to  go  back  a  stage  and  investigate  the 
origin  of  the  emanation.  We  shall  first  consider  an  important 
series  of  experiments  by  Rutherford  and  Soddy,1  which  have 
not  only  solved  this  question,  but  also  have  thrown  a  strong 
light  on  the  processes  occurring  in  thorium. 

A  small  quantity  of  thorium  nitrate  was  taken  and  dissolved 
in  water.  Sufficient  ammonia  was  then  added  to  precipitate 
the  thorium  present  as  hydroxide.  The  filtrate  remaining  was 
then  evaporated  to  .  dryness,  and  the  ammonium  salts  driven  off 
by  heating.  The  small  residue  finally  obtained  was,  weight 
for  weight,  over  one  thousand  times  as  active  as  the  original 
thorium  nitrate.  The  great  activity  of  this  residue,  as  com- 
pared with  ordinary  thorium,  can  be  readily  illustrated  by  means 
of  the  electroscope.  The  active  residue  obtained  from  50  grams 
of  the  nitrate  causes  the  gold  leaves  of  the  electroscope  to  collapse 
in  a  few  seconds,  while  a  weight  of  thorium  nitrate  equal  to 
that  of  the  residue  causes  hardly  an  appreciable  movement. 

1  Rutherford  and  Soddy :  Phil.  Mag.,  Sept.  and  Nov.,  1902. 


RADIOACTIVE   CHANGES   IN   THORIUM         57 

The  active  substance  present  in  this  residue  was  called  for 
convenience,  thorium  X  (ThX).  It  probably  exists  in  almost 
infinitesimal  quantity  mixed  with  the  impurities  left  behind 
after  the  evaporation  of  the  reagent,  together  possibly  with  a 
small  trace  of  thorium  which  escaped  precipitation.  Since 
ThX  is  derived  from  the  thorium  salt,  the  latter  must  have 
been  deprived  of  some  of  its  activity.  This  was  found  to  be 
the  case,  for  the  thorium  hydroxide,  so  separated,  had  only 
about  half  of  the  activity  to  be  normally  expected. 

The  a  ray  activity  of  the  ThX  and  the  precipitated  hydroxide 
were  examined  at  intervals  by  means  of  an  electrometer.  The 
activity  of  ThX  was  not  permanent,  but  increased  for  the  first 
day  and  then  decayed  exponentially,  falling  to  half  value  in 
about  4  days.  After  a  month's  interval,  the  activity  sank  to 
a  minute  fraction  of  its  original  value.  The  curve  showing 
the  variation  of  activity  of  ThX  with  time  is  seen  in  Fig.  16, 
Curve  I. 

Now  let  us  turn  our  attention  to  the  precipitated  hydroxide. 
The  activity  of  this  decreased  to  some  extent  during  the  first 
day,  passed  through  a  minimum,  and  then  steadily  increased 
again  with  the  time,  reaching  an  almost  steady  value  after 
a  month's  interval.  These  results  are  shown  in  Fig.  16, 
Curve  II. 

The  two  curves  of  decay  of  ThX,  and  of  recovery  of  activity 
of  the  thorium,  bear  a  very  simple  relation  to  one  another.  The 
initial  rise  in  the  ThX  curve  is  seen  to  correspond  to  a  fall  in 
the  recovery  curve  of  the  thorium,  and  when  the  activity  of 
ThX  has  almost  disappeared  the  activity  of  the  thorium  has 
practically  reached  a  maximum  value.  The  sum  of  the  activi- 
ties of  the  ThX  and  of  the  thorium  from  which  it  was  separated 
is  very  nearly  constant  over  the  whole  range  of  the  experiment. 
The  two  curves  of  recovery  and  decay  are  complementary  to 
each  other.  As  fast  as  the  ThX  loses  its  activity,  the  thorium 
regains  it.  This  relation  between  the  curves  is,  at  first  view, 
most  remarkable,  and  it  would  appear  as  if  there  were  some 
mutual  influence  between  the  ThX  and  the  thorium  compound 
from  which  it  was  removed,  so  that  the  latter  absorbed  the 


58 


RADIOACTIVE   TRANSFORMATIONS 


activity  lost  by  the  former.  This  position  is,  however,  quite 
untenable,  for  the  rise  and  recovery  curves  are  independent,  and 
are  unaltered  if  the  thorium  and  ThX  are  kept  in  sealed  vessels 
far  removed  from  each  other.  If  the  thorium  hydroxide  after 


FIG.  16. 
Decay  of  activity  of  thorium  X  and  recovery  of  activity  of  thorium  deprived  of  ThX. 

recovering  its  activity  is  again  dissolved  and  ammonia  added, 
the  amount  of  ThX  separated  is  found  to  be  the  same  as  thai 
obtained  from  the  first  experiment.  This  process  can  be  repeate< 
indefinitely,  and  equal  quantities  of  ThX  always  be  separatee 


RADIOACTIVE   CHANGES   IN   THORIUM         59 

provided  that  about  a  month  elapses  between  each  precipitation 
in  order  to  allow  the  thorium  to  regain  its  lost  activity.  This 
shows  that  there  is  a  fresh  growth  of  ThX  in  the  thorium  after 
each  precipitation. 

We  shall  now  consider  the  explanation  of  the  connection 
between  the  decay  and  recovery  curves.  For  the  moment  we 
shall  disregard  the  initial  irregularities  shown  by  the  two  curves, 
which  will  be  discussed  later.  If  the  recovery  curve  of  Fig.  16 


20 


12  16 

Time  in  Days 

FIG.  17. 

Decay  curve  of  thorium  X  and  recovery  curve  of  thorium,  from  measurements 
one  day  after  removal  of  the  thorium  X. 


is  produced  backwards  to  cut  the  vertical  axis,  it  does  so  at 
a  minimum  of  25  per  cent.  The  curve  of  recovery  of  the  lost 
activity  reckoned  from  this  25  per  cent  minimum  is  shown  in 
Fig.  17.  In  the  same  figure  is  shown  the  decay  curve  of  ThX 
beginning  after  the  second  day  and  plotted  to  the  same  scale. 
The  decay  curve  of  the  ThX  is  exponential,  decreasing  to  half 


60  RADIOACTIVE   TRANSFORMATIONS 

value  in  about  4  days.    The  decrease  of  activity  from  the  initial 
value  /0  is  given  by  the  equation 


The  two  curves  are  complementary,  and  the  sum  of  the  or- 
dinates  at  any  time  is  equal  to  100  on  the  arbitrary  scale.  After 
4  days,  the  activity  of  ThX  has  decayed  to  half  value,  and  in 
the  same  interval  the  thorium  has  regained  half  its  lost  activity. 
The  recovery  curve  is  thus  expressed  by  an  equation  of  the  form 


where  It  is  the  activity  recovered  after  any  time  t,  and  IQ  the 
maximum  activity  which  is  regained  when  a  steady  state  is 
reached.  In  this  equation  X  has  exactly  the  same  value  as  for 
the  decay  curve. 

Following  the  same  line  of  argument  employed  to  interpret 
the  decay  curve  of  the  emanation  (page  43),  we  may  suppose 
that  the  ThX  is  an  unstable  substance  which  is  half  transformed 
in  4  days,  the  amount  of  ThX  breaking  up  per  second  being 
always  proportional  to  the  amount  present.  The  radiation  con- 
sisting of  a  rays  accompanies  the  change,  and  is  also  proportional 
to  the  amount  of  ThX  present. 

Now  we  have  seen  that  fresh  ThX  is  produced  in  the  thorium 
after  the  first  supply  has  been  removed.  This  production  of 
ThX  proceeds  at  a  constant  rate,  but  the  amount  of  ThX 
present  in  the  thorium  cannot  increase  indefinitely,  for  at  the 
same  time  the  ThX  is  being  changed  continuously  into  another 
substance.  A  steady  state  will  obviously  be  reached  when  the 
rate  of  production  of  new  ThX  exactly  compensates  for  the 
rate  of  disappearance  of  ThX  due  to  its  own  transformation. 
Now  the  number  of  atoms  of  ThX  which  break  up  per  second 
is  equal  to  \N  where  X  is  the  radioactive  constant  of  ThX,  and 
N  is  the  number  of  atoms  of  ThX  present  at  any  time. 

A  steady  state  is  reached  when  the  number  q  o£  atoms  of 


RADIOACTIVE   CHANGES   IN   THORIUM         61 


fresh  ThX  supplied  per  second  is  equal  to  the  number 
which  break  up  per  second,  where  N0  is  the  maximum  number 
present  when  equilibrium  is  reached;  i.  e., 


At  any  time  the  rate  of  increase   —  —  of  the  number  of  atoms 

ctt 

of  ThX  present  is  equal  to  the  difference  between  the  rate  of 
supply  and  the  rate  of  disappearance;  i.  e., 

dN 


The  solution  of  this  equation  is  of  the  form 

N=  a  e~™  +  b, 

where   a   and    b   are   constants.      Since   N  =  0  when  t  =  0, 
a  +  b  =0,  and  remembering  that  when  £  =  oo  ,  Nis  equal  to  JV0, 

a  =  —  b  =  —  N0, 
and  consequently 

N 

-^  =  1  —  e     - 

^o 

This  theoretical  equation  expressing  the  number  of  atoms 
of  ThX  present  at  any  time  is  thus  identical  in  form  with  the 
equation  of  variation  of  activity  obtained  experimentally.  We 
therefore  see  that  the  decay  and  recovery  curves  of  ThX  are 
completely  explained  on  the  simple  hypotheses  :  — 

(1)  That  there  is  a  constant  production  of  fresh  ThX  from 
the  thorium; 

(2)  That  the  ThX  is  continuously  transformed,  the  amount 
changing  per  second  being  always  proportional  to  the  amount 
present. 

The  hypothesis  (2)  has  been  previously  shown  to  be  merely 
another  method  of  expression  of  the  observed  exponential  law 
of  decay  of  the  activity  of  ThX. 

The   first  hypothesis   can   be   proved   experimentally.     The 


62  RADIOACTIVE   TRANSFORMATIONS 

amount  N  of  ThX  present  after  the  growth  has  been  continued 
for  a  time  t  should  be  given  by 


where  NQ  is  the  equilibrium  amount. 

Since  the  ThX  is  half  transformed  in  4  days,  \  =  .173  (day)"1. 
At  the  end  of  1  day  after  complete  removal  of  the  ThX,  the 
amount  formed  consequently  should  be  16  per  cent  of  the  maxi- 
mum; after  4  days,  50  per  cent;  after  8  days,  75  per  cent,  and 
so  on.  Now  it  was  found  experimentally  that  three  rapid  pre- 
cipitations of  thorium  by  ammonia  almost  completely  freed  it 
from  ThX  for  the  time  being.  After  standing  for  definite 
periods,  the  ThX  present  was  removed  and  the  amounts  ob- 
tained were  found  to  be  in  good  agreement  with  the  theory. 

We  thus  see  that  the  apparent  constant  radioactivity  of 
thorium  is  really  the  result  of  two  opposing  processes  of  growth 
and  decay ;  for  radioactive  matter  is  being  continuously  formed, 
and  this  matter  in  turn  is  continuously  changing,  and  conse- 
quently losing  its  activity.  There  is  thus  a  type  of  chemical 
equilibrium  in  which  the  rate  of  production  of  new  matter 
balances  the  rate  at  which  the  new  matter  is  transformed. 

SOURCE  OF  THE  THORIUM  EMANATION 

A  thorium  compound  completely  freed  from  ThX  gives  off 
very  little  emanation,  even  in  a  state  of  solution.  On  the  other 
hand,  the  ammonia  solution  which  contains  the  ThX  gives  off 
a  large  amount.  The  removal  of  ThX  is  thus  accompanied  by 
the  removal  of  the  emanating  power  of  thorium.  It  seems  prob- 
able, therefore,  that  the  emanation  is  derived  from  ThX,  and 
further  experiment  has  proved  this  to  be  the  case.  If  a  solution 
of  ThX  is  taken,  and  a  constant  stream  of  air  bubbled  through 
it,  the  amount  of  emanation  liberated  is  found  to  decrease  expo- 
nentially, falling  to  half  value  in  4  days.  This  is  exactly  the 
result  to  be  expected  if  ThX  is  the  parent  of  the  emanation, 
for  the  activity  of  ThX  is  a  measure  of  the  number  of  atoms  of 
ThX  breaking  up  per  second,  i.  e.,  a  measure  of  the  number 


RADIOACTIVE   CHANGES   IN   THORIUM         6B 

of  atoms  of  the  new  substance  which  is  formed.  The  rate  of 
production  of  emanation  by  the  ThX  should,  on  this  view,  be 
always  proportional  to  the  activity  of  ThX,  and  consequently 
should  diminish  at  the  same  rate  and  according  to  the  same 
law.  This,  as  we  have  seen,  has  been  experimentally  observed. 
Although  the  thorium  after  removal  of  ThX  is  for  the  time 
almost  entirely  deprived  of  the  power  of  emitting  an  emanation, 
this  property  is  gradually  regained,  according  to  the  same  law 
as  the  recovery  law  of  ThX  shown  in  Fig.  17.  This  result 
follows  naturally  if  ThX  is  the  parent  of  the  emanation.  The 
emanating  power  should  be  proportional  to  the  amount  of  ThX 
present,  and  should  consequently  vary  pari  passu  with  it. 

We  may  thus  conclude  with  confidence  that  the  property  of 
emitting  an  emanation  is  not  a  direct  property  of  thorium  itself, 
but  belongs  to  its  product  ThX. 

INITIAL  IRREGULARITIES  IN  THE  DECAY  AND 
RECOVERY  CURVES 

We  are  now  in  a  position  to  explain  the  initial  irregularities 
in  the  decay  and  recovery  curves  shown  in  Fig.  16.  The 
activity  of  the  separated  ThX  at  first  increases,  while  the 
activity  of  the  precipitated  thorium  at  first  diminishes.  Now 
the  active  deposit  produced  from  the  emanation  is  insoluble  in 
ammonia,  and  consequently  is  left  behind  with  the  thorium. 
The  ThX  after  separation  produces  the  emanation,  and  this,  in 
turn,  is  transformed  into  thorium  A  and  B.  The  activity  sup- 
plied by  thorium  B  more  than  compensates  at  first  for  the  decay 
of  the  activity  of  ThX  alone.  The  activity  consequently  rises, 
but  since  the  rates  of  transformation  of  A  and  B  are  rapid  com- 
pared with  that  of  ThX,  after  about  one  day  equilibrium  is 
practically  reached,  when  very  nearly  the  same  number  of  atoms 
of  ThX,  and  of  each  of  its  products,  break  up  per  second. 
When  this  is  the  case  the  activity  of  the  emanation  and  of 
thorium  B  will  vary  exactly  in  the  same  way  as  that  of  the 
parent  substance  ThX.  The  activity  of  the  active  residue  — 
which  is  a  measure  of  the  activity  due  to  ThX,  the  emanation, 


64  RADIOACTIVE   TRANSFORMATIONS 

and  thorium  B  together  —  will  in  consequence  decrease  expo- 
nentially, falling  to  half  value  in  four  days. 

Since  the  active  deposit  produced  by  the  emanation  in  the 
mass  of  the  thorium  compound  is  not  removed  with  the  ThX, 
the  activity  due  to  it  must  at  first  diminish,  for,  in  the  ab- 
sence of  ThX  and  the  emanation,  there  is  no  fresh  supply 
of  thorium  A  and  B  to  compensate  for  their  transformation. 
The  activity  of  the  thorium  will  thus  diminish  until  the  fresh 
supply  of  activity  due  to  ThX  and  its  succeeding  products  com- 
pensates for  the  decrease  in  the  activity  of  the  deposit.  The 
activity  will  then  be  at  a  minimum,  and  will  afterwards  increase 
with  the  time,  in  consequence  of  the  continued  production 
of  ThX. 

The  complementary  character  of  the  curves  of  decay  and 
recovery,  quite  apart  from  the  special  considerations  here  ad- 
vanced, is  a  necessary  consequence  of  the  laws  governing 
radioactive  changes.  The  rate  of  transformation,  so  far  as 
observation  has  gone,  is  not  affected  by  physical  and  chemical 
conditions.  The  transformation  of  ThX,  when  mixed  with 
thorium,  takes  place  at  the  same  rate,  and  according  to  the 
same  laws,  as  when  it  is  isolated  from  the  thorium  by  a  chemical 
process.  When  the  activity  of  a  thorium  compound  has  reached 
a  constant  value,  the  activity  is  then  due  to  the  various  active 
products  formed  in  it.  If  a  product  is  separated  by  chemical 
or  other  means  from  the  thorium  compound,  the  activity  due 
to  this  product  plus  that  due  to  the  thorium  and  the  active 
products  left  behind,  must  be  equal  to  the  constant  value  of 
the  activity  of  the  original  thorium  in  equilibrium.  This  follows 
at  once,  for  otherwise  there  would  be  a  creation  or  destruction 
of  radioactivity  by  the  mere  removal  of  one  of  the  products,  and 
this  would  involve  a  gain  or  loss  of  radioactive  energy.  If,  as  in 
the  case  of  ThX,  the  separated  product  first  increases  and  then 
decreases  in  activity,  there  must  be  a  corresponding  decrease 
followed  by  an  increase  in  the  activity  of  the  thorium  from 
which  it  has  been  separated,  in  order  that  the  sum  of  the  activ- 
ities of  the  two  may  be  constant. 

This  principle  of  the  conservation  of  the  total  amount  of 


RADIOACTIVE   CHANGES   IN   THORIUM         65 

radioactivity  applies  not  only  to  thorium,  but  to  radioactive 
substances  generally.  The  total  radioactivity  of  any  substance 
in  equilibrium  cannot  be  altered  by  any  physical  or  chemical 
agency,  although  the  radioactivity  may  be  manifested  in  a  series 
of  products,  capable  of  separation  from  the  parent  substance. 
There  is  reason  to  believe,  however,  that  the  radioactivity  of 
the  primary  active  substances  is  not  strictly  permanent,  but 
diminishes  slowly,  although  in  the  case  of  feebly  active  elements 
like  uranium  and  thorium,  probably  no  appreciable  change 
would  be  detected  in  a  million  years. 

With  an  intensely  active  body  like  radium,  it  will  be  shown 
later  that  in  all  probability  the  sum  total  of  the  activity  will 
ultimately  decay  exponentially,  decreasing  to  half  value  in  about 
thirteen  hundred  years.  Provided,  however,  that  the  period 
of  observation  is  small  compared  with  the  life  of  the  primary 
substance,  the  principle  of  the  constancy  of  the  radioactivity 
is  a  sufficiently  accurate  expression  of  the  experimental  results. 
Many  examples  in  support  of  this  principle  will  be  found  in 
succeeding  chapters  of  this  book. 

METHODS  OF  SEPARATION  OF  THORIUM  PRODUCTS 

In  addition  to  ammonia,  several  reagents  have  been  found 
capable  of  removing  ThX  from  thorium  solutions.  Schlundt 
and  R.  B.  Moore 1  found  that  pyridine  and  fumaric  acid  separate 
ThX  from  thorium  nitrate  solutions.  These  reagents  differ 
from  ammonia  in  removing  the  inactive  product,  thorium  A, 
with  the  ThX,  while  the  active  product,  thorium  B,  is  left 
behind  with  the  thorium. 

Von  Lerch 2  has  shown  that  ThX  can  be  separated  by  elec- 
trolysis from  an  alkaline  solution  of  ThX,  using  amalgamated 
zinc,  copper,  mercury,  or  platinum  as  electrodes.  The  period 
of  ThX  has  been  accurately  determined,  and  found  to  be  3.64 
days.  In  addition,  Von  Lerch  found  that  ThX  was  deposited 
on  different  metals  by  leaving  them  for  several  hours  in  an 
alkaline  solution  of  ThX.  Iron  and  zinc  removed  the  greatest 

1  Schlundt  and  R.  B.  Moore:  Journ.  Phys.  Chem.,  Nov.,  1905. 

2  Von  Lerch:  Wien.  Ber.,  March,  1905. 

5 


66  RADIOACTIVE   TRANSFORMATIONS 

quantity.  A  nickel  plate  dipped  into  an  acid  solution  of  the 
active  deposit  becomes  coated  with  thorium  B,  for  the  activity 
observed  on  the  metal  decays  exponentially  with  a  period  of 
1  hour.  Other  metals  similarly  treated  also  became  active,  but 
their  rates  of  decay  show  that  they  are  coated  with  a  mixture 
of  thorium  A  and  of  thorium  B. 

These  results  have  shown  in  a  striking  way  the  differences 
in  physical  and  chemical  properties  of  the  various  thorium  prod- 
ucts. The  methods  of  separation  of  the  infinitesimal  quan- 
tities of  matter  present  are  as  definite  as  the  ordinary  chemical 
methods,  applied  to  matter  existing  in  considerable  amount, 
while  the  radiating  property  serves  as  a  simple  and  reliable 
method  of  qualitative  and  quantitative  analysis. 

CHANGES  IN  THORIUM 

We  have  so  far  shown  that  thorium  produces  ThX,  and  that 
the  latter  is  transformed  into  the  emanation,  which  undergoes 
two  further  changes  into  thorium  A  and  thorium  B. 

If  thorium  is  subjected  to  a  succession  of  precipitations  with 
ammonia,  extending  over  several  days,  the  ThX  is  removed  as 
fast  as  it  is  formed,  and  the  active  deposit  has  time  to  disap- 
pear. The  activity  of  the  thorium  then  sinks  to  a  minimum 
of  25  per  cent  of  its  value  when  in  equilibrium.  The  recovery 
curve  of  the  thorium  treated  in  this  way  does  not  show  the 
initial  decrease  already  referred  to,  but  rises  steadily,  according 
to  the  recovery  curve  shown  in  Fig.  17.  It  is  thus  seen  that 
thorium  itself  supplies  only  25  per  cent  of  the  total  a  ray  activ- 
ity of  thorium  when  in  equilibrium,  and  that  the  rest  is  due  to 
ThX,  the  emanation,  and  thorium  B.  Each  of  these  a  ray  prod- 
ucts supplies  about  25  per  cent  of  the  total  activity.  Such  a 
result  is  to  be  expected,  for,  when  in  equilibrium,  an  equal 
number  of  atoms  of  thorium  ThX,  the  emanation,  and  thorium 
B  must  break  up  per  second.  This  is  based  on  the  reasonable 
supposition,  that  each  atom  in  breaking  up  gives  rise  to  one 
atom  of  the  succeeding  product.  The  results,  so  far  obtained, 
are  completely  explained  on  the  disintegration  theory  put  for- 
ward by  Rutherford  and  Soddy.  On  this  theory,  a  minute  con- 


RADIOACTIVE   CHANGES   IN   THORIUM 


67 


slant  fraction  of  the  atoms  of  thorium  becomes  unstable  every 
second,  and  breaks  up  with  the  expulsion  of  an  a  particle.  The 
residue  of  the  atom  after  the  loss  of  an  a  particle  becomes  an 
atom  of  a  new  substance,  thorium  X.  This  is  far  more  unstable 
than  the  thorium  itself,  and  breaks  up  with  the  expulsion  of  an 
a  particle,  half  the  matter  being  transformed  in  4  days.  ThX  in 
turn  changes  into  the  emanation,  which  again  breaks  up  into  the 
active  deposit,  consisting  of  two  successive  products,  thorium  A 
and  thorium  B.  The  atom  of  thorium  B  breaks  up  with  the  ac- 
companiment of  an  a  and  a  /3  particle,  and  7  rays.  Thorium  A 
is  transformed  into  thorium  B  without  the  appearance  of  rays. 
Such  a  change  may  consist  either  of  a  rearrangement  of  the  parts 
constituting  the  atom  without  the  projection  of  a  part  of  its 
mass,  or  of  the  expulsion  of  an  a  particle  at  too  low  a  velocity 
to  ionize  the  gas.  From  the  considerations  advanced  later  in 
Chapter  X,  the  latter  supposition  does  not  appear  improbable. 

A  table  of  the  products  of  thorium  and  some  of  their  charac- 
teristic physical  and  chemical  properties  is  given  below. 

TABLE  OF  TRANSFORMATION  PRODUCTS  OF  THORIUM. 


Radioactive 
product. 

Time  to  be  half 
transformed. 

Nature  of 
rays. 

Some  physical  and  chemical  properties. 

Thorium 

About  109  years 

a 

Insoluble  in  ammonia. 

Thorium  X 

4  days 

a 

Separated  from  thorium  by  its 

solubility  in  ammonia  and  in 

water  and  by  electrolysis  ;  sep- 

arated also  by  f  umaric  acid  and 

> 

» 

pyridine. 

Emanation 

54  sees. 

a 

A  chemically  inert  gas  of  high 

molecular  weight  ;    condenses 

from  gases  at  a  temperature  of 

> 

f 

-120°  C. 

Thorii 

N 

im  A 

r 

11  hours 

No  rays'l 

Deposited  on  the  surface  of  bod- 
ies ;  concentrated  on  the  nega- 
tive   electrode   in  an    electric 

Thorium  B 

1  hour 

«"  &  7    •) 

field  ;  soluble  in  strong  acids  ; 

volatilized    at    high    tempera- 

tures.   A  is  more  volatile  than 

B.   A  can  be  separated  from  B 

by  electrolysis  and  by  its  dif- 

\ 

' 

r 
I 

ference  in  volatility. 

68  RADIOACTIVE   TRANSFORMATIONS 

The  family  of  products  of  thorium  is  graphically  represented 
in  Fig.  18. 

RADIOTHORIUM 

There  has  been  a  considerable  difference  of  opinion  as  to 
whether  thorium  is  a  true  radioactive  element  or  not,  i.  e.,  as 
to  whether  the  activity  of  thorium  is  due  to  thorium  itself,  or  to 
some  active  substance  normally  always  associated  with  it.  Some 
experimenters  state  that  by  special  methods  they  have  obtained 
an  almost  inactive  substance  giving  the  chemical  tests  of  tho- 
rium. Some  recent  work  of  Hahn  J  is  of  especial  importance 
in  this  connection. 


THORIUM.        rnoK.x.      E/viflNariON.    THOR.A.     THOR.B. 

FIG.  18. 
Family  of  thorium  products. 

Working  with  the  Ceylon  mineral,  thorianite,  which  consists 
mainly  of  thorium  and  12  per  cent  of  uranium,  Hahn  was  able 
by  special  chemical  methods  to  separate  a  small  amount  of  a 
substance  comparable  in  activity  with  radium.  This  substance, 
which  has  been  named  "  radiothorium, "  gave  off  the  thorium 
emanation  to  such  an  intense  degree  that  the  presence  of  the 
emanation  could  be  easily  seen  by  the  luminosity  produced  on  a 
zinc  sulphide  screen.  Thorium  X  could  be  separated  from  it  in 
the  same  way  as  from  thorium,  while  the  excited  activity  pro- 
duced by  the  emanation  decayed  with  the  period  of  11  hours 
characteristic  for  thorium.  The  activity  of  radiothorium  seems 
to  be  fairly  permanent,  and  it  seems  probable  that  this  active 

i  Hahn  :  Proc.  Roy.  Soc.,  March  16, 1905  ;  Jahrbuch.  d.  Radioaktivitat,  II,  Heft  3. 
1905. 


RADIOACTIVE   CHANGES   IN   THORIUM         69 

substance  is  in  reality  a  lineal  product  of  thorium  intermediate 
between  thorium  and  thorium  X.  The  radiothorium  produces 
thorium  X,  which  in  turn  produces  the  emanation.  It  still 
remains  to  be  shown  that  this  active  substance  can  be  separated 
from  ordinary  thorium,  but  there  can  be  little  doubt  that  radio- 
thorium  is  either  the  active  constituent  mixed  with  thorium,  or, 
what  is  more  probable,  that  it  is  a  product  of  thorium.  We 
shall  see  later  that  actinium  itself  is  inactive,  although  it  gives 
rise  to  a  succession  of  active  products  remarkably  similar  in 
many  respects  to  the  family  of  products  observed  in  thorium. 
The  results  of  Hahn  suggest  that  the  transformation  of  thorium 
itself  may  be  rayless,  but  that  the  succeeding  product,  radio- 
thorium,  gives  out  rays.  Further  results  are  required  before 
such  a  conclusion  can  be  considered  as  definitely  established,  but 
the  results  so  far  obtained  by  Hahn  are  of  the  greatest  interest 
and  importance. 


CHAPTER  III 
THE   RADIUM  EMANATION 

SHORTLY  after  the  writer l  had  shown  that  thorium  compounds 
continuously  emit  a  radioactive  emanation,  Dorn2  found  that 
radium  possesses  a  similar  property.  Very  little  emanation 
is  emitted  from  radium  compounds  in  a  solid  state,  but  it 
escapes  freely  when  the  radium  is  dissolved  or  heated.  While 
the  emanations  of  thorium  and  radium  possess  very  analogous 
properties,  they  can  readily  be  distinguished  from  each  other 
by  the  difference  in  the  rates  of  decay  of  their  activities.  While 
the  activity  of  the  thorium  emanation  decreases  to  half  value 
in  54  seconds,  and  practically  disappears  in  the  course  of  10 
minutes,  that  of  the  radium  emanation  is  far  more  persistent, 
for  it  takes  nearly  4  days  to  be  reduced  to  half  value,  and  is 
•still  appreciable  after  a  month's  interval. 

In  physical  and  chemical  properties  the  radium  emanation 
is  very  similar  to  that  of  thorium,  but,  on  account  of  its  great 
activity  and  comparatively  slow  rate  of  change,  it  has  been 
.studied  in  more  detail  than  the  latter.  It  has  been  found  possi- 
ble to  isolate  it  chemically  and  to  measure  its  volume,  as  well 
as  to  observe  its  spectrum.  The  activity  and  concomitant  heat- 
ing effect,  which  are  enormous  in  comparison  with  the  amount  of 
matter  involved,  have  drawn  strong  attention  to  this  substance, 
for  the  effects  produced  are  of  a  magnitude  that  can  neither  be 
easily  explained  nor  explained  away.  For  these  reasons,  we  shall 
consider  in  some  detail  the  more  important  chemical  and  physical 
properties  of  the  radium  emanation,  and  the  connection  that 
exists  between  them.  The  study  of  this  substance  will  throw 
additional  light  on  the  general  theory  of  radioactivity  which  has 
already  been  developed  in  the  last  chapter. 

1  Rutherford:  Phil.  Mag.,  Jan.,  Feb.,  1900. 

2  Dorn:  Naturforsch.  Ges.  fur  Halle  a.  S.,  1900. 


THE   RADIUM   EMANATION  71 

The  salts  of  radium,  generally  employed  in  experimental 
work,  are  the  bromide  and  the  chloride.  Both  of  these  com- 
pounds emit  very  little  emanation  into  a  dry  atmosphere.  The 
emanation  produced  is  stored  up  or  occluded  in  the  mass  of  the 
substance,  but  is  released  by  heating  or  dissolving  the  compound. 
The  enormous  activity  of  the  emanation  set  free  from  radium  is 
very  well  illustrated  by  the  following  simple  experiment. 

A  minute  crystal  of  the  bromide  or  chloride  is  dropped  into  a 
small  wash  bottle.  A  few  cubic  centimeters  of  water  are  added 
to  dissolve  the  compound,  and  the  bottle  is  immediately  closed. 
A  slow  current  of  air  is  then  sent  through  the  solution,  and  is 
carried  along  a  narrow  glass  tube  into  the  interior  of  an  elec- 
troscope. If  the  electroscope  is  initially  charged  the  leaves  are 
observed  to  collapse  almost  immediately  after  the  air  reaches  it. 
It  is  then  found  impossible  to  cause  a  divergence  of  the  leaves  for 
more  than  a  moment.  If  the  emanation  is  all  blown  out  from 
the  electroscope  by  a  current  of  air,  the  leaves  are  still  observed 
to  collapse  rapidly,  although  the  emanation  has  been  completely 
removed. 

This  residual  activity  is  due  to  an  active  deposit  left  on  the 
sides  of  the  vessel.  In  this  respect,  the  emanation  of  radium 
possesses  a  similar  property  to  that  of  thorium.  The  activity, 
however,  diminishes  more  rapidly  than  in  the  case  of  thorium, 
for  most  of  the  electrical  effect  due  to  it  disappears  in  a  few 
hours,  while  in  the  case  of  thorium  the  effect  lasts  for  several 
days. 

Measurements  of  the  rate  of  decay  of  the  activity  of  the  ema- 
nation have  been  made  by  several  observers.  Rutherford  and 
Soddy *  stored  a  quantity  of  air  mixed  with  emanation  in  a  small 
gasometer  over  mercury,  and  a  definite  volume  was  withdrawn 
at  intervals  and  discharged  into  a  testing  vessel  such  as  is  shown 
in  Fig.  10.  The  activity  observed  in  the  vessel  increased  for 
several  hours  after  the  introduction  of  the  emanation  on  account 
of  the  formation  of  the  active  deposit.  By  determining  the 
saturation  current  immediately  after  the  passage  of  the  emana- 
tion into  the  testing  cylinder,  the  quantity  of  emanation  initially 

1  Rutherford  and  Soddy:  Phil.  Mag.,  April,  1903. 


72  RADIOACTIVE   TRANSFORMATIONS 

present  was  measured.  In  this  way  it  was  found  that  the  amount 
of  emanation  present  decreased  according  to  an  exponential  law, 
falling  to  half  value  in  3.77  days.  P.  Curie  1  determined  the 
constant  of  decay  of  the  emanation  in  a  somewhat  different  way. 
A  large  quantity  of  emanation  was  introduced  into  a  glass  tube, 
which  was  then  sealed  off,  and  the  ionization  due  to  the  issuing 
rays  was  measured  at  intervals  by  an  electrometer  in  a  suitable 
testing  vessel.  Now  it  will  be  seen  later  that  the  emanation  gives 
out  only  a  rays,  which  are  completely  stopped  by  a  thickness  of 
glass  less  than  ^  mm. ;  consequently  the  rays  from  the  emana- 
tion were  absorbed  in  the  walls  of  the  glass  tube.  The  electrical 
effect  produced  in  the  testing  vessel  was  due  entirely  to  the  ft 
and  7  rays  which  are  emitted  from  the  active  deposit  produced 
on  the  inside  of  the  tube  by  the  emanation.  Since  after  about 
3  hours  the  active  deposit  is  in  radioactive  equilibrium  with 
the  emanation,  and  then  decays  at  the  same  rate  as  the  parent 
substance,  the  intensity  of  the  /3  and  7  rays  will  diminish  at  the 
same  rate  and  according  to  the  same  law  as  the  emanation  itself. 
In  this  way  the  activity  was  found  to  diminish  according  to  an 
exponential  law,  falling  to  half  value  in  3.99  days.  The  agree- 
ment of  these  periods  of  decay  obtained  by  different  methods 
shows  that  the  amount  of  the  active  deposit  is  always  propor- 
tional to  the  amount  of  emanation  present  at  any  time  during  the 
life  of  the  emanation.  This  is  one  of  the  proofs  that  the  active 
deposit  is  a  product  of  the  decomposition  of  the  emanation. 

Further  experiments  to  determine  the  constant  of  decay  of 
the  emanation  have  been  made  by  Bumstead  and  Wheeler,2  and 
Sackur.3  The  former  found  that  the  activity  decreased  to  half 
value  in  3.88  days,  and  the  latter  found  the  period  to  be  3.8.6 
days.  We  may  thus 'conclude  that  the  emanation  decays  ex- 
ponentially with  a  period  of  about  3.8  days. 

The  emanation  from  radium  is  almost  entirely  released 
boiling  a  solution  of  the  compound  or  by  aspirating  air  througl 
it.     The  active  deposit  is  left  behind  with  the  radium,  but  thii 

1  P.  Curie:  Comptes  rendus,  cxxxv,  p.  857  (1902).         ' 

2  Bumstead  and  Wheeler  :  Amer.  Jour.  Science,  Feb.,  1904. 

8  Sackur:  Ber.  d.  d.  chem.  Ges.,  xxxviii,  No.  7,  p.  1754  (1905). 


THE   RADIUM   EMANATION 


73 


disappears  after  several  hours.  If  the  radium  solution  is  then 
evaporated  to  dryness,  the  activity  measured  by  the  a  rays  is 
found  to  have  reached  a  minimum  of  about  25  per  cent  of  the 
normal  value.  If  kept  in  a  dry  atmosphere,  the  emanation  pro- 
duced from  the  radium  is  occluded  in  its  mass,  and  the  activity 
of  the  radium  consequently  increases,  reaching  its  normal  steady 
value  after  about  one  month.  The  recovery  curve  of  the  activ- 
ity of  radium  from  the  25  per  cent  minimum  is  shown  in  Fig.  19. 
The  decay  curve  of  the  emanation  is  added  for  comparison. 


/oo 


FIG.  19. 

Decay  curve  of  the  radium  emanation  and  recovery  curve  of  the  activity  of  radium 
measured  by  the  a  rays,  from  the  25  per  cent  minimum. 

As  in  the  case  of  thorium,  the  decay  and  recovery  curves 
are  complementary  to  each  other.  The  activity  of  the  emana- 
tion falls  to  half  value  in  about  3.8  days,  while  half  of  the  lost 
activity  of  the  radium  is  recovered  in  the  same  interval. 

The  activity  of  the  emanation  released  from  the  radium  is 
thus  given  at  any  time  by  the  equation 


74  RADIOACTIVE   TRANSFORMATIONS 

while  the  equation  of  the  recovery  curve  from  the  minimum  is 

h  -  I  _ 
' 


i.  e.,  the  amount  of  the  emanation  N  stored  up  in  the  radium 
after  standing  for  a  time  t  is  given  by 


where  N0  is  the  maximum  amount.  These  curves  are  explained 
in  exactly  the  same  way  as  the  similar  curves  for  thorium.  The 
emanation  is  an  unstable  substance  which  is  half  transformed 
in  3.8  days.  It  is  produced  at  a  constant  rate  by  the  radium, 
and  the  activity  of  the  radium  reaches  a  steady  value  when  the 
rate  of  production  of  fresh  emanation  balances  the  rate  of  dis- 
appearance of  that  already  formed. 

A  radium  compound  initially  freed  from  emanation  will  have 
grown  a  maximum  supply  again  about  one  month  later,  and  this 
process  of  removal  and  fresh  growth  may  be  continued  indefi- 
nitely. If  NO  be  the  number  of  atoms  of  emanation  presenl 
^vvhen  in  equilibrium,  the  rate  q  of  supply  of  fresh  atoms  oi 
emanation  by  the  radium  is  equal  to  the  number  lost  by  its  owi 
decomposition,  i.  e., 


The  value  of  X  thus  has  a  definite  physical  meaning,  for  i1 
represents  the  fraction  of  the  equilibrium  amount  of  emanatioi 
supplied  per  second,  as  well  as  the  fraction  of  the  emanatioi 
which  breaks  up  per  second.  Taking  the  period  of  the  emana- 
tion as  3.8  days,  the  value  of  X,  with  the  second  as  the  unit  oi 
time  is  1/474000,  or,  in  other  words,  the  rate  of  supply  of  the 
emanation  per  second  is  1/474000  of  the  equilibrium  amounl 

This  result  is  well  illustrated  by  a  very  simple  experimenl 
described   by   Rutherford   and   Soddy.      A   small   quantity  oi 
radium  chloride   in  radioactive  equilibrium  was  dropped  in  I 
hot  water.     The  accumulated  emanation  released  by  solutioi 


THE   RADIUM  EMANATION  75 

was  swept  with  a  current  of  air  into  a  suitable  testing  vessel, 
and  the  saturation  current  immediately  measured.  The  current 
so  determined  is  a  comparative  measure  of  JVi,  the  equilibrium 
amount  of  emanation  stored  up  in  the  radium. 

The  radium  solution  was  then  aspirated  with  air  for  some 
time,  to  remove  the  last  trace  of  accumulated  emanation,  and 
then  allowed  to  stand  undisturbed  for  105  minutes.  The 
emanation  accumulated  in  this  interval  was  then  swept  into  a 
similar  testing  vessel  and  the  saturation  current  again  deter- 
mined. This  current  is  a  measure  of  the  amount  Nt  of  the 

N 

emanation   formed   in   the   interval.     The  ratio  —  was  found 

to  be  .0131,  and  disregarding  the  small  decay  of  the  emanation 
during  such  a  short  interval, 

Nt  =  q  X  105  X  60. 
It  follows  that  -j=r  =  1/480000. 

Allowing  for  the  small  decay  during  the  interval, 
•jf  =  1/477000. 

From  the  constant  of  decay  of  the  emanation  we  have  seen 
that 

A  =  J^  =  1/474000. 

The  agreement  between  theory  and  experiment  is  thus  re- 
markably close,  and  is  a  direct  proof  that  the  production  of 
emanation  in  a  solid  compound  proceeds  at  the  same  rate  as  in 
the  solution.  In  the  former  case  it  is  occluded,  and  in  the 
latter,  part  is  retained  in  the  solution  and  the  rest  in  the  air 
space  above  it. 

It  is  surprising  how  tenaciously  the  emanation  is  held  by  dry 
radium  compounds.  Experiment  showed  that  the  emanating 
power  in  the  solid  state  was  less  than  one  half  per  cent  of  the 
emanating  power  in  solution.  Since  a  radium  compound  stores 


76  RADIOACTIVE   TRANSFORMATIONS 

up  nearly  500,000  times  as  much  emanation  as  is  produced  per 
second,  the  result  shows  that  the  amount  of  emanation  escaping 
per  second  is  less  than  one  hundred  millionth  part  of  that  oc- 
cluded in  the  compound.  The  rate  of  escape  of  emanation  is 
much  increased  in  a  moist  atmosphere  and  by  rise  of  temperature. 

The  recovery  curve  of  a  solid  radium  compound  freed  from 
emanation  is  altered  if  the  conditions  allow  much  of  the  recov- 
ered emanation  to  escape.  Under  such  conditions,  the  maximum 
activity  is  reached  more  quickly,  and  is  far  smaller  than  the 
normal  activity  of  a  non-emanating  compound. 

This  property  of  radium  of  retaining  its  emanation  is  difficult 
to  explain  satisfactorily  unless  it  is  assumed  that  there  is  some 
slight  chemical  combination  between  the  emanation  and  the 
radium  producing  it.  Godlewski l  has  suggested  that  the  eman- 
ation is  in  a  state  of  solid  solution  with  the  parent  matter. 
This  point  of  view  is  supported  by  certain  observations  made  by 
him  on  the  rapidity  of  diffusion  of  the  product  uranium  X  into 
a  uranium  compound.  A  discussion  of  his  results  will  be  given 
later  in  Chapter  VII. 

CONDENSATION  OF  THE  EMANATION 

For  several  years  after  the  discovery  of  the  emanations  from 
thorium  and  radium,  there  existed  considerable  difference  of 
opinion  as  to  their  real  nature.  Some  physicists  suggested 
that  they  were  not  material,  but  consisted  of  centres  of  force 
attached  to  the  molecules  of  gas  with  which  the  emanation  was 
mixed,  and  moving  with  them.  Others  held  that  the  emana- 
tion was  a  gas  present  in  such  minute  amount  that  it  was  diffi- 
cult to  detect  by  means  of  the  spectroscope  or  by  direct  chemical 
methods.  The  objections  urged  against  the  material  character 
of  the  emanation  were  to  a  large  extent  removed  by  the  dis- 
covery, made  by  Rutherford  and  Soddy,2  that  the  emanations 
of  thorium  and  radium  possessed  a  characteristic  property  of 
gases  inasmuch  as  they  could  be  condensed  from  the  inactive 
gas  with  which  they  were  mixed  by  the  action  of  extreme  cold. 

1  Godlewski:  Phil.  Mag.,  July,  1905. 

2  Rutherford  and  Soddy:  Phil.  Mag.,  May,  1903. 


THE   RADIUM   EMANATION 


77 


As  a  result  of  a  careful  series  of  experiments,  it  was  found  that 
the  emanation  from  radium  condensed  at  a  temperature  of 
—150°  C.  The  condensation  and  volatilization  points  were  very 
sharply  defined,  and  did  not  differ  by  more  than  1°  C.  The 
thorium  emanation  commenced  to  condense  at  about— 120°  C., 
but  the  condensation  was  not  usually  completed  until  a  tempera- 
ture of  —150°  C.  was  reached.  The  probable  cause  of  this  in- 
teresting difference  in  behavior  of  the  two  emanations  will  be 
discussed  presently. 

If  a  large  amount  of  emanation  is  available,  the  condensation 
of  the  radium  emanation  can  readily  be  followed  by  the  eye. 
The  experimental  arrangement  is  clearly  shown  in  Fig.  20. 

The  emanation  mixed 
with  air  is  stored  in  a 
small  gasometer,  and 
is  then  slowly  passed 
through  a  U  tube  im- 
mersed in  liquid  air. 
This  U  tube  is  filled 
with  fragments  of  wil- 
lemite, or  crystals  of 
barium  platinocyanide, 
which  become  luminous 
under  the  influence  of 
the  rays  from  the  ema- 
nation. If  the  current  of  air  mixed  with  emanation  is  passed 
very  slowly  through  the  tube,  the  fragments  of  willemite  begin 
to  glow  brightly  just  below  the  level  of  the  liquid  air,  and  the 
luminosity  can  be  concentrated  over  a  short  length  of  the  tube. 
This  shows  that  the  emanation  has  been  condensed  at  the  tem- 
perature of  liquid  air,  and  is  deposited  on  the  walls  of  the  tube 
and  on  the  surface  of  the  willemite.  If  the  U  tube  is  then 
partially  exhausted  and  closed  with  stopcocks,  the  emanation 
still  remains  concentrated  for  some  minutes  on  the  tube  and 
willemite,  although  the  liquid  air  is  removed.  When,  however, 
the  temperature  of  the  tube  rises  to  —150°  C.,  the  emanation  is 
rapidly  volatilized,  and  distributed  throughout  the  tube.  This 


RADIUM    EMANATION 


FIG.  20. 


78  RADIOACTIVE   TRANSFORMATIONS 

is  observed  by  the  sudden  distribution  of  the  luminosity  through- 
out the  whole  mass  of  willemite  in  the  U  tube.  The  point  of 
condensation  remains  brighter  than  the  rest  of  the  tube  for  some 
time.  This  is  due  to  the  fact  that  the  emanation,  even  in  the 
condensed  state,  has  produced  the  active  deposit.  When  the 
emanation  is  volatilized,  the  active  deposit  remains  behind,  and 
the  rays  from  it  cause  a  greater  luminosity  at  that  point.  After 
an  hour's  interval  this  difference  of  luminosity  has  almost  dis- 
appeared, and  the  willemite  glows  throughout  with  a  uniform 
light.  The  luminosity  can  at  any  time  be  concentrated  at  any 
point  by  local  cooling  with  liquid  air. 

If  the  U  tube  is  filled  with  different  layers  of  phosphorescent 
materials,  like  willemite,  kunzite,   zinc  sulphide,   and  barium 
platinocyanide,    the   emanation   after  volatilization   is   equally 
distributed,  and  each  layer  of  material  glows  with  its  own  pecu 
liar  light.    The  greenish  luminosity  of  the  willemite  and  barium 
platinocyanide  is  not  easily  distinguishable,  except  for  a  differ 
ence  of  intensity.     The  kunzite  glows  with  a  deep  red  color 
while  the  zinc  sulphide  emits  a  yellow  light.     There  are  severa 
interesting  points  of  distinction  between  the  action  of  the  ray 
of  the  emanation  and  of  the  active  deposit  on  these  substances 
Unlike  the  other  substances  mentioned,  the  luminosity  of  zinc 
sulphide  largely  disappears  at  the  temperature  of  liquid  air,  bu 
revives  at  a  higher  temperature.     The  a  rays  produce  a  markec 
luminosity  in  willemite,  the  platinocyanides,  and  zinc  sulphide 
but  have  little  or  no  effect  in  lighting  up  kunzite.    The  latter  i 
sensitive  only  to  the  /3  and  7  rays  emitted  from  the  active  deposit 
In  consequence  of  this,  the  kunzite  is  very  feebly  luminous  when 
the  emanation  is  first  introduced.    The  light,  however,  increases 
in  intensity  as  the  active  deposit  is  produced  by  the  emanation 
and  reaches  a  maximum  about  three  hours  after  the  introduction 
of  the  emanation.    After  barium  platinocyanide  has  been  exposec 
for  some  time  to  the  action  of  a  large  amount  of  the  emanation 
the  crystals  change  to  a  reddish  tinge,  and  the  luminosity  is 
much  reduced.     This  has  been  shown  to  be  due  to  a  permanen 
change  in  the  crystals  by  the  action  of  the  rays.    By  re-solution 
and  crystallization,  the  luminosity  again  returns. 


THE   RADIUM   EMANATION 


79 


Curie  and  Debierne  early  showed  that  glass  becomes  luminous 
under  the  action  of  the  rays  from  the  emanation.  This  effect 
is  most  marked  in  Thuringian  glass,  but  as  a  rule  the  luminosity 
is  feeble  compared  with  that  produced  in  willemite  or  zinc  sul- 
phide. The  glass  becomes  colored  under  the  action  of  the  rays, 
and  with  strong  emanation  is  rapidly  blackened. 

The  sharpness  of  the  temperature  of  volatilization  of  the 
radium  emanation  was  very  clearly  illustrated  by  some  experi- 
ments made  by  Rutherford  and  Soddy,  using  the  electric  method. 


FIG.  21. 

Determination  of  the  temperature  of  condensation  of  the  radium  emanation 
by  the  electric  method. 

The  emanation  collected  in  the  gasometer,  B,  was  condensed  in 
a  long  spiral  copper  tube,  S,  (see  Fig.  21)  immersed  in  liquid 
air,  and  a  slow  steady  stream  of  air  after  passing  through  the 
tube  entered  a  small  testing  vessel,  T.  After  condensation  the 
copper  spiral  was  removed  from  the  liquid  air  and  allowed  to 
heat  up  very  slowly.  The  temperature  was  deduced  from 
measurements  of  the  resistance  of  the  copper  spiral.  Just  be- 
fore the  point  of  volatilization  was  reached,  very  little  effect 
was  observed  in  the  testing  vessel.  Suddenly  a  rapidly  increas- 
ing movement  of  the  electrometer  needle  was  noted,  and  by 


80  RADIOACTIVE   TRANSFORMATIONS 

using  a  large  quantity  of  emanation  the  rate  of  movement  in- 
creased in  a  few  moments  from  several  divisions  to  several 
hundred  divisions  per  second.  The  rise  of  temperature  ob- 
served between  the  point  at  which  there  was  practically  no 
escape  of  the  emanation  and  the  point  of  rapid  escape  was  not 
more  than  a  fraction  of  a  degree  in  many  cases. 

It  has  been  already  pointed  out  that  the  temperature  of  com- 
plete condensation  of  the  thorium  emanation  is  not  at  all  sharp, 
but  that  the  condensation  in  most  cases  continues  over  a  range  of 
about  30°  C.  This  striking  difference  in  the  behavior  of  the  two 
emanations  is,  in  all  probability,  due  to  the  small  amount  of  the 
thorium  emanation  present  in  the  experiments.  The  emanation 
of  thorium  breaks  up  at  about  six  thousand  times  the  rate  of 
that  of  radium.  For  an  equal  expulsion  of  a  particles  by  the  two 
emanations,  i.  e.  for  approximately  equal  electrical  effects,  the 
latter  must  therefore  be  present  in  at  least  six  thousand  times  the 
amount  of  the  former.  In  addition,  in  most  of  the  experiments 
with  the  radium  emanation,  the  quantity  of  emanation  was  suffi- 
cient to  produce  several  hundred  times  the  electrical  effect  ob- 
served with  the  small  quantity  of  the  emanation  obtained  from 
thorium  compounds.  Thus,  in  some  of  the  experiments,  the 
quantity  of  radium  emanation  present  was  at  least  ten  thousand 
times  —  and  in  many  cases  more  than  a  million  times  —  the 
amount  of  the  thorium  emanation.  In  fact,  it  can  readily  be  cal- 
culated that  in  the  actual  experiments  not  more  than  100  atoms 
of  thorium  emanation  could  have  been  present  per  cubic  centi- 
metre of  gas  carried  through  the  copper  spiral.  Under  such 
conditions,  it  is  not  so  much  a  matter  of  surprise  that  the 
emanation  of  thorium  does  not  show  a  sharp  condensation  point, 
as  that  the  emanation  can  be  condensed  at  all  when  so  sparsely 
distributed  throughout  a  volume  of  gas. 

Diminution  of  the  pressure  of  the  air  in  the  spiral,  or  the  sub- 
stitution of  hydrogen  for  oxygen  as  the  carrying  medium,  both 
tended  to  cause  more  rapid  condensation.  Such  an  effect  is  to 
be  expected  on  the  above  view,  since  the  rapidity  of  diffusion 
^f  the  atoms  of  emanation  through  the  gas  is  thereby  hastened. 

If  the  thorium  emanation  should  ever  be  obtained  in  large 


THE   RADIUM   EMANATION  81 

quantity,  there  can  be  little  doubt  that  it  will  also  exhibit  com- 
paratively sharp  points  of  condensation  and  volatilization.  The 
fact  that  the  thorium  emanation  begins  to  condense  at  a  higher 
temperature  (—120°  C.)  than  the  radium  emanation  (—150°  C.) 
shows  that  the  emanations  consist  of  different  types  of  matter. 
The  emanation  of  actinium,  like  the  emanations  from  radium  and 
thorium,  may  be  condensed  by  passing  it  through  a  spiral  im- 
mersed in  liquid  air,  but  the  rapidity  of  the  decay  of  its  activity 
(half  value  in  3.9  seconds)  makes  an  accurate  determination  of 
its  condensation  temperature  by  the  electric  method  very  diffi- 
cult, since  the  emanation  would  lose  the  greater  part  of  its  activ- 
ity before  the  stream  of  gas  carrying  the  emanation  could  be 
reduced  to  the  temperature  of  the  spiral.  The  ease  with  which 
the  radium  emanation  is  condensed  by  liquid  air  has  proved  of 
great  importance  in  many  recent  researches  on  the  emanation. 
By  the  use  of  this  property,  it  has  been  freed  from  the  gases  mixed 
with  it,  isolated  in  a  pure  state,  and  its  spectrum  determined. 

RATE  OP  DIFFUSION  OF  THE  EMANATION 

If  the  emanation  is  introduced  at  one  end  of  a  tube  kept  at 
constant  temperature,  after  the  lapse  of  several  hours  it  is  found 
to  be  distributed  in  equal  amount  throughout  the  volume  of  the 
tube.  This  result  shows  that  the  emanation  diffuses  through 
the  air  like  an  ordinary  gas.  It  has  not  yet  been  found  possible 
to  determine  by  a  direct  method  the  density  of  the  emanation, 
as  the  quantity  released  from  even  one  gram  of  pure  radium 
bromide  would  be  too  small  to  be  weighed  accurately.  By  com- 
paring the  rate  of  diffusion  of  the  emanation  with  that  of  a 
known  gas,  we  can,  however,  obtain  a  rough  estimate  of  its 
molecular  weight.  The  rates  of  interdiffusion  of  various  gases 
have  long  been  known  to  decrease  with  the  molecular  weight 
of  the  diffusing  gas.  If  therefore,  for  example,  we  find  that  the 
coefficient  of  interdiffusion  of  the  emanation  into  air  lies  between 
the  corresponding  values  obtained  for  two  known  gases,  A  and 
B,  it  is  probable  that  the  molecular  weight  of  the  emanation  is 
intermediate  in  value  between  that  of  A  and  B. 

Shortly  after  the  discovery  of  the  radium  emanation,  Ruther- 

6 


82  RADIOACTIVE   TRANSFORMATIONS 

ford  ancMNIiss  Brooks 1  determined  its  coefficient  of  interdiffu- 
sion  K  into  air,  and  found  values  lying  between  K  =  .07  and 
K=  .09.  The  method  adopted  was  to  divide  a  long  cylinder 
into  equal  parts  by  a  movable  slide.  The  emanation  was  first 
introduced  into  one  half  of  the  tube,  and  thoroughly  mixed 
with  the  air.  When  temperature  conditions  were  steady,  the 
slide  was  opened,  and  the  emanation  gradually  diffused  into  the 
other  half.  The  amount  %of  emanation  present  in  each  half  of 
the  tube,  at  any  time  after  opening  the  slide,  was  determined 
by  the  electric  method,  and  from  these  data  the  coefficient  of 
interdiffusion  can  be  calculated.  The  coefficient  of  interdiffu- 
sion  of  carbon  dioxide  (molecular  weight  44)  into  air  was  long 
ago  found  to  be  .142.  The  emanation  thus  diffuses  into  air 
more  slowly  than  does  carbon  dioxide  into  air.  For  alcohol 
vapor  (molecular  weight  77),  the  value  of  .2"  =.077.  Taking 
the  lower  value,  K=  .07,  as  the  more  probable  value  for  the 
radium  emanation,  it  follows  that  the  emanation  has  a  molecu- 
lar weight  greater  than  77. 

A  number  of  determinations  have  since  been  made,  by  differ- 
ent methods,  to  form  an  estimate  of  the  molecular  weight  of  the 
emanation. 

Bumstead  and  Wheeler2  measured  directly  the  comparative 
rates  of  diffusion  of  the.  emanation  and  of  carbon  dioxide  through 
a  porous  pot.  Assuming  Graham's  law,  viz.,  that  the  coeffi- 
cient of  interdiffusion  is  inversely  proportional  to  the  square 
root  of  the  molecular  weight,  they  deduced  that  the  molecular 
weight  of  the  emanation  was  about  172. 

Makower,3  using  a  similar  method,  compared  the  rates  of 
diffusion  of  the  radium  emanation  through  a  porous  pot  with  the 
rates  for  the  gases  oxygen,  carbon  dioxide,  and  sulphur  dioxide, 
and  finally  concluded  that  the  emanation  had  a  molecular  weight 
in  the  neighborhood  of  100.  Curie  and  Danne 4  determined  the 

1  Rutherford  and  Miss  Brooks :  Trans.  Roy.  Soc.,  Canada,  1901 ;  Chemical  News, 
1902, 

2  Bumstead  and  Wheeler  :  Amer.  Journ.  Sci.,  Feb.,  1904. 
»  Makower:  Phil.  Mag.,  Jan.,  1905. 

*  Curie  and  Danne  :  Comptes  rendus,  cxxxvi,  p.  1314  (1904). 


THE   RADIUM   EMANATION  83 

rate  of  diffusion  of  the  emanation  through  capillary  tubes,  and 
obtained  a  value  K  =  .09,  a  value  somewhat  higher  than  that 
obtained  by  Miss  Brooks  and  the  writer. 

It  is  thus  seen  that  all  the  experiments  on  diffusion  bear 
out  the  conclusion  that  the  emanation  is  a  heavy  gas  with  a 
molecular  weight  probably  not  less  than  100.  It  is  doubt- 
ful, however,  whether  much  reliance  can  be  placed  on  the 
actual  value  of  the  molecular  weight  deduced  in  this  way, 
because  the  emanation  exists  in  minute  amount  in  the  gas 
in  which  it  diffuses,  and  its  coefficient  of  interdiffusion  is 
compared  with  that  of  gases  existing  in  large  quantity.  The 
coefficients  of  interdiffusion  may  not  in  such  a  case  be  directly 
comparable.  In  addition,  the  rate  of  diffusion  of  the  eman- 
ation, which  has  the  properties  of  a  monatomic  gas,  is  coin- 
pared  with  the  rates  of  diffusion  of  gases  which  have  complex 
molecules. 

If  the  emanation  is  considered  to  be  a  direct  product  of  radium, 
and  to  consist  of  the  radium  atom  minus  one  or  two  a  particles, 
the  molecular  weight  should  be  not  much  less  than  the  atomic 
weight  of  radium,  viz.,  225.  It  is  doubtful  whether  the  value 
of  the  molecular  weight  of  the  emanation  can  be  determined 
with  any  certainty  until  the  emanation  has  been  obtained  in 
sufficient  amount  to  determine  its  density. 

The  coefficient  of  interdiffusion  of  the  thorium  emanation 
into  air  has  been  determined  by  the  writer  to  be  about  .09. 
This  would  suggest  that  the  thorium  emanation  has  a  somewhat 
smaller  molecular  weight  than  that  of  radium. 

The  emanation  obeys  £he  laws  of  gases,  not  only  as  regards 
diffusion,  but  also  in  other  particulars.  For  example,  the 
emanation  divides  itself  between  two  connected  reservoirs  in 
proportion  to  their  volumes.  P.  Curie  and  Danne  showed 
that  if  one  of  the  reservoirs  was  kept  at  a  temperature  of 
10°  C.  and  the  other  at  350°  C.,  the  emanation  is  distributed 
between  them  in  the  same  proportion  as  a  gas  under  the  same 
conditions. 

The  emanation  thus  possesses  the  characteristic  properties  of 
gases,  namely,  condensation  and  diffusion.  It  also  obeys  at  low 


84  RADIOACTIVE   TRANSFORMATIONS 

temperatures  Charles's  law,  and,  as  will  be  seen  later,  Boyle's 
law. 

We  may  thus  conclude  with  confidence  that  the  emanation, 
while  it  exists,  is  a  radioactive  gas  of  heavy  molecular  weight. 

PHYSICAL  AND  CHEMICAL  PROPERTIES  OF  THE  EMANATION 

A  number  of  experiments  have  been  made  to  determine 
whether  the  emanation  possesses  any  definite  chemical  proper- 
ties which  would  enable  us  to  compare  it  with  any  other  known 
gas,  but  so  far  no  evidence  has  been  obtained  that  the  emana- 
tion is  able  to  combine  with  other  substances.  In  such  experi- 
ments, the  electric  method  offers  a  simple  and  accurate  method 
of  determining  whether  the  quantity  of  the  emanation  is  re- 
duced under  various  conditions.  In  fact,  it  serves  as  a  rapid 
and  exact  method  of  quantitative  analysis  of  the  minute  amount 
of  emanation  under  experiment. 

Rutherford  and  Soddy  l  showed  that  the  emanation  was  not 
diminished  in  quantity  after  condensation  by  liquid  air,  or  by 
passage  through  a  platinum  tube  kept  at  a  white  heat  by  an 
electric  current.  A  number  of  experiments  were  also  made  in 
which  the  emanation  was  made  to  pass  over  a  number  of  re- 
agents, the  emanation  being  always  mixed  with,  a  gas  unaffected 
by  the  particular  reagent.  They  concluded  from  these  experi- 
ments that  no  gas  could  have  survived  in  unaltered  amount  the 
severe  treatment  to  which  it  had  been  exposed,  except  an  inert 
gas  of  the  helium-argon  family. 

Ramsay  and  Soddy2  found  that  the  quantity  of  emanation 
was  unchanged  after  sparking  for  several  •  hours  with  oxygen 
over  alkali.  The  oxygen  was  then  removed  by  ignited  phos- 
phorus, and  no  visible  residue  was  left.  Another  gas  was  then 
introduced,  and  the  emanation  after  mixture  with  it  was  with- 
drawn. Its  activity  was  practically  unaltered.  A  similar  re- 
sult was  observed  when  the  emanation  was  introduced  into  a 
magnesium  lime  tube  which  was  heated  for  three  hours  to 
a  red  heat. 

1  Rutherford  and  Soddy:  Phil.  Mag.,  Nov.,  1902. 

2  Ramsay  and  Soddy :  Proc.  Roy.  Soc.,  Ixxii,  p.  204  (1903). 


THE   RADIUM   EMANATION  85 

We  may  thus  conclude  that  the  radium  emanation  in  respect 
to  the  absence  of  definite  combining  properties  is  allied  to  the 
recently  discovered  inert  gases  of  the  atmosphere.  On  the  dis- 
integration theory,  the  emanation  is  supposed  to  be  transformed 
with  the  accompaniment  of  the  expulsion  of  a  particles.  It  is 
of  great  importance  to  settle  whether  the  rate  of  disintegration 
is  affected  by  temperature.  Any  change  in  the  rate  of  trans- 
formation would  result  in  a  change  in  the  period  of  decay  of 
the  emanation.  This  point  has  been  examined  by  P.  Curie, 
who  found  that  the  decay  of  activity  was  unaffected  by  con- 
Atinued  exposure  of  the  emanation  to  temperatures  varying  be- 
/tween  -180°  C.  and  450°  C. 

This  result  shows  that  the  transformation  of  the  emanation 
cannot  be  considered  to  be  a  type  of  ordinary  chemical  dissocia- 
tion, for  no  reaction  is  known  in  chemistry  which  is  independ- 
ent of  temperature  over  such  a  wide  range.  In  addition,  the 
transformation  of  the  emanation  is  accompanied  by  the  expul- 
sion of  a  portion  of  its  mass  at  enormous  speed  —  a  result  never 
observed  in  chemical  reactions.  Such  a  result  suggests  that 
the  change  that  occurs  is  not  molecular  but  atomic.  This  view 
is  strongly  confirmed  by  the  enormous  release  of  energy  during 
the  disintegration  of  the  emanation  which  will  be  considered 
later. 

VOLUME  OP  THE  EMANATION 

It  has  been  seen  that  the  amount  of  emanation  to  be  obtained 
from  a  given  quantity  of  radium  reaches  a  maximum  value 
when  the  rate  of  supply  of  fresh  emanation  balances  the  rate  of 
transformation  of  that  already  produced.  Since  this  maximum 
amount  of  emanation  is  always  proportional  to  the  quantity  of 
radium  present,  the  volume  of  emanation  released  from  one 
gram  of  radium  in  radioactive  equilibrium  should  have  a  definite 
constant  value.  It  was  early  recognized  that  the  volume  of  the 
emanation  to  be  obtained  from  one  gram  of  radium  was  very 
small,  but  not  too  minute  to  be  measured.  From  the  data  avail- 
able at  the  time,  the  writer l  in  1903  calculated  that  the  volume 

1  Rutherford  :  Nature,  Aug.  20,  1903  ;  Phil.  Mag.,  Aug.,  1905. 


86  RADIOACTIVE   TRANSFORMATIONS 

of  the  emanation  derived  from  one  gram  of  radium  probably  lay 
between  .06  and  .6  cubic  millimetres  at  atmospheric  pressure 
and  temperature. 

A  more  accurate  deduction  can  be  made  from  the  more  recent 
experimental  data  of  the  number  of  a  particles  expelled  from 
one  gram  of  radium  per  second.  This  number  has  been  deter- 
mined by  the  writer  by  measuring  the  positive  charge  communi- 
cated to  a  body  on  which  the  a  rays  impinged.  Assuming  that 
each  a  particle  carries  an  ionic  charge  of  3.4  x  10~10  electro- 
static units,  it  was  deduced  that  one  gram  of  radium  at  its  mini- 
mum activity  (i.  e.,  when  the  emanation  and  its  disintegration 
products  were  removed)  emitted  6.2  x  1010  a  particles  per  second. 
If  we  suppose,  as  is  probably  the  case,  that  each  radium  atom 
in  breaking  up  gives  rise  to  one  atom  of  the  emanation,  the 
number  of  atoms  of  emanation  produced  per  second  is  equal  to 
the  number  of  a  particles  expelled  per  second. 

But  NO,  the  maximum  number  of  atoms  of  emanation  stored 

up  in  radium  in  radioactive  equilibrium,  is  given  by  NQ  =  |> 

A 

where  q  is  the  rate  of  production  and  X  is  the  decay  constant. 

Consequently,  the  value  of  No  for  one  gram  of  radium  is 
6.2  x  ID"  x  474,000,  or  2.94  x  1016. 

Now  from  experimental  data  it  is  known  that  one  cubic  cen- 
timetre of  any  gas  at  atmospheric  pressure  and  temperature 
contains  3.6  x  1019  molecules.  Assuming  that  the  molecule 
of  the  emanation  consists  of  one  atom,  the  volume  of  emanation 
from  one  gram  of  radium  is 

9  Q9  v  1 016 

36x10"  =  .0008  c.c.,  or  0.8  c.mms. 

We  shall  now  consider  the  changes  that  may  be  expected  to 
occur  in  a  volume  of  pure  emanation  from  the  point  of  view  of 
the  disintegration  theory.  The  emanation  emits  a  particles 
and  is  transformed  into  the  active  deposit,  which  behaves  as 
a  type  of  non-gaseous  matter  and  attaches  itself  to  the  walls 
of  the  containing  vessel.  The  amount  of  emanation  decreases 
exponentially,  falling  to  half  value  in  3.8  days.  We  should 


THE   RADIUM   EMANATION 


87 


thus  expect  the  volume  of  the  emanation  to  shrink,  and  since 
the  activity  of  the  emanation  has  decayed  to  a  small  fraction  of 
its  original  value  after  one  month,  the  volume  of 
the  emanation  after  that  interval  should  be  very 
small.  The  remarkable  way  in  which  these  theo- 
retical conclusions  have  been  verified  will  now  be 
considered. 

Ramsay  and  Soddy l  attacked  the  difficult  prob- 
lem of  isolating  the  emanation  and  determining 
its  volume  in  the  following  way.  The  emanation 
from  60  milligrams  of  radium  bromide  in  solu- 
tion was  collected  for  8  days, 
and  then  drawn  off  through 
the  inverted  siphon,  E  (see 
Fig.  22),  into  the  explosion 
burette,  F.  The  radium  in 
the  solution  produces  hydro- 
gen and  oxygen  at  a  rapid 
rate,  and  the  emanation  was 
initially  removed  with  these 
gases.  After  explosion,  the 
slight  excess  of  hydrogen 
mixed  with  the  emanation  was 
left  for  some  time  in  contact 
with  caustic  soda,  placed  in 
the  upper  part  of  the  burette, 
in  order  to  remove  the  car- 
bon dioxide  present.  In  the 
meantime,  the  upper  part  of 
the  apparatus  had  been  ex- 
hausted as  completely  as  possi- 
ble. The  connection  with  the 
mercury  pump  was  then  closed, 
and  the  hydrogen  and  ema- 
nation allowed  to  enter  the  apparatus,  passing  over  a  phos- 
phorus pentoxide  tube,  D,  to  remove  all  trace  of  water  vapor. 

1  Kamsay  and  Soddy  :  Proc.  Koy.  Soc.,  Ixxiii.,  p.  346  (1904). 


FIG.  22. 

Apparatus  of  Kamsay  and  Soddy  for 

determining  the  volume  of  the 

radium  emanation. 


88 


RADIOACTIVE  TRANSFORMATIONS 


The  emanation  was  condensed  in  the  lower  part  of  the  tube  B, 
which  was  surrounded  by  liquid  air.  The  process  of  condensa- 
tion of  the  emanation  at  B  was  made  evident  by  the  brilliant 
luminosity  of  the  lower  part  of  the  tube.  The  mercury  of  the 
burette  was  allowed  to  run  to  A,  and  the  tube  AB  again  com- 
pletely pumped  out.  The  connection  with  the  pump  was  again 
closed,  the  liquid  was  removed,  and  the  volatilized  emanation 
forced  into  the  accurately  calibrated  capillary  tube  A.  Obser- 
vations were  then  made  for  a  space  of  several  weeks  on  the 
variation  in  volume  of  the  emanation.  The  results  are  shown 
in  the  following  table : 


Time. 

Volume.                                Time. 

Volume. 

Start 

0.124   c.mms. 

7  days 

0.0050  c.mms. 

Iday 

0.027    "     " 

9 

0.0041  "     « 

3 

0.011    "     " 

11 

0.0020  "     " 

4 

0.0095  "     « 

12 

0.0011  «     " 

6 

0.0063  "     " 

The  volume  decreased,  and  after  four  weeks  only  a  minul 
bubble  remained,  but  this  retained  its  luminosity  to  the  Ias1 
During  this  time,  the  tube  was  colored  a  deep  purple  by  th< 
rays.     This  caused  difficulties  in  readings  of  the  volume,  and 
strong  source  of  light  was  found  necessary.    Ramsay  and  Soddj 
consider  that  the  apparent  sudden  decrease  during  the  first  da] 
may  have  been  due  to  the  mercury  sticking  in  the  capillary  tube. 
Taking  the  readings  after  one  day,  the  volume  of  the  emanatioi 
is  found  to  shrink  approximately  according  to  an  exponent^ 
law,  decreasing  to  half  value  in  about  4  days.     This  is  aboul 
the  rate  of  decrease  of  volume  to  be  expected  from  theoretic* 
considerations.      Another  experiment  was  made  with  a  fresl 
supply  of  emanation,  but  a  very  surprising  difference  was  note( 
The  gas  had  an  initial  volume  of  0.0254  c.mm.  at  atmospheri< 
pressure,  and  a  special  series  of  experiments  was  made  to  detei 
mine  the  volume  occupied  by  the  gas  in  the  capillary  tube 
varying  pressures.     The  emanation  was  found  to  obey  Boyle's 
law  within  the  limit  of  experimental  error.     Unlike  its  behavic 
in  the  first  experiment,  however,  the  volume  occupied  by  the 


THE   RADIUM   EMANATION  89 

in  the  capillary  tube,  instead  of  shrinking,  steadily  increased, 
and  23  days  later  was  about  10  times  the  initial  value.  At  the 
same  time,  bubbles  commenced  to  appear  in  the  mercury  column 
below  the  level  of  the  gas. 

Further  experiments  are  necessary  in  order  to  elucidate  the 
contradictions  observed  in  these  two  experiments.  It  will  be 
seen  later  that  the  gas  helium  is  a  transformation  product  of 
the  emanation.  This  appears  to  have  been  absorbed  in  the 
walls  of  the  tube  in  the  first  experiment.  Such  a  result  is  not 
unexpected,  for  there  is  considerable  evidence  that  the  a  parti- 
cles expelled  from  the  radioactive  products  consist  of  helium 
atoms  projected  with  great  velocity.  Most  of  these  atoms 
would  be  buried  in  the  walls  of  the  glass  tube  to  an  average 
depth  of  about  .02  mm.,  and  their  diffusion  back  into  the  gas 
may  depend  on  the  kind  of  glass  employed.  The  most  plausi- 
ble explanation  is  that  the  helium  after  absorption  by  the  walls 
of  the  glass  capillary  diffused  back  into  the  gas  in  the  second 
experiment,  but  not  in  the  first. 

Ramsay  and  Soddy  concluded  from  their  experiments  that 
the  maximum  volume  of  the  emanation  released  from  one  gram 
of  radium  was  slightly  greater  than  one  cubic  millimetre  at 
standard  pressure  and  temperature. 

The  theoretical  and  calculated  amounts  0.8  and  1  c.  mm., 
respectively,  are  thus  in  very  good  agreement,  and  indicate  the 
general  correctness  of  the  theory  on  which  the  calculations  are 
based. 

SPECTRUM  OF  THE  EMANATION 

After  the  isolation  of  the  emanation,  and  determination  of  its 
volume,  a  number  of  experiments  were  made  by  Ramsay  and 
Soddy  to  determine  its  spectrum.  In  some  of  the  experiments 
several  apparently  new  bright  lines  were  seen  for  a  moment, 
but  these  rapidly  vanished  in  consequence  of  the  liberation  of 
hydrogen  in  the  tube.  Ramsay  and  Collie  l  continued  the  ex- 
periments, and  were  finally  successful  in  obtaining  the  spectrum 
of  the  emanation,  which  lasted  for  a  sufficient  interval  to  deter- 
mine rapidly  the  wave-lengths  of  the  more  obvious  lines  by 

1  Ramsay  and  Collie:  Proc.  Roy.  Soc.,  Ixxiii.,  p.  470  (1904). 


90  RADIOACTIVE   TRANSFORMATIONS 

means  of  eye  measurements.  The  spectrum,  however,  soon 
faded,  and  was  finally  completely  masked  by  that  of  hydrogen. 
They  state  that  the  spectrum  was  very  brilliant  and  consisted 
of  a  number  of  bright  lines,  the  spaces  between  being  perfectly 
dark.  The  spectrum  bore  a  striking  resemblance  in  general 
character  to  the  spectra  of  the  inert  gases  of  the  argon  family. 
On  repeating  the  experiment  with  a  fresh  supply  of  emanation, 
many  of  the  bright  lines  were  seen  again,  while  some  new  lines, 
not  observed  in  the  first  spectrum,  made  their  appearance. 
They  conclude  that  the  emanation  undoubtedly  has  a  definite 
and  well-marked  spectrum  of  bright  lines. 

HEAT  EMISSION  OF  THE  EMANATION 

One  gram  of  radium  in  radioactive  equilibrium  continuously 
emits  heat  at  the  rate  of  about  100  gram  calories  per  hour.  If 
the  emanation  is  released  from  the  radium  by  solution  or  heat- 
ing, the  heating  effect  of  the  radium  decreases  to  a  minimum  of 
about  25  per  cent  of  the  original  value,  and  then  as  new  ema- 
nation is  formed  it  gradually  increases,  reaching  its  old  value 
after  a  month's  interval.  The  vessel  containing  the  emanation 
released  from  the  radium  is  found  to  emit  heat  at  a  rapid  rate, 
and,  three  hours  after  removal,  gives  out  about  75  per  cent  of 
the  heat  emitted  by  the  original  radium.  The  rate  of  heat 
emission  of  the  emanation  decays  at  the  same  rate  as  it  loses 
its  activity,  i.  e.,  it  falls  to  half  value  in  about  4  days.  Th< 
curves  of  decrease  of  heating  effect  of  the  emanation  and  oi 
recovery  of  the  heating  effect  of  the  radium  are,  like  the  activ- 
ity curves,  complementary  to  each  other.  The  heat  emissioi 
of  the  two  together  is  always  equal  to  that  of  the  radium 
radioactive  equilibrium. 

The  heat  emission  of  the  tube  containing  the  emanation 
not  due  to  the  emanation  alone,  but  also  to  the  active  deposit 
formed  from  the  emanation.  The  laws  controlling  the  heat 
emission  of  radium  and  its  products  will  be  considered  moi 
completely  in  Chapter  X. 

It  is  thus  seen  that  the  emanation,  together  with  its  trans 
formation  products,  is  responsible  for  about  three  quarters 


THE   RADIUM   EMANATION  91 

the  heat  emission  of  radium.  It  is  difficult  to  disentangle  the 
heating  effect  of  the  emanation  from  that  of  its  rapidly  chang- 
ing products,  but  there  is  no  doubt  that  it  supplies  about  one 
quarter  of  the  total  heating  effect  of  the  radium. 

Thus  one  cubic  millimetre  of  the  emanation  —  the  maximum 
amount  released  from  one  gram  of  radium  —  itself  emits  heat  at 
the  rate  of  25  gram  calories  per  hour.  Now  the  heating  effect 
of  the  emanation  falls  off  at  the  same  rate  as  its  activity.  The 
total  heat  emission  of  the  emanation  during  its  life  is  given 
by  Ql\.  The  value  of  X,  with  the  hour  as  the  unit  of  time, 
is  1/132,  and  since  Q  =  25,  the  total  heat  emitted  by  the  emana- 
tion is  3300  gram  calories.  If  we  include  with  that  of  the 
emanation  the  heating  effects  of  its  subsequent  products,  the 
total  heat  emitted  from  the  emanation  tube  is  about  three  times 
this  amount,  or  9900  gram  calories.  This  corresponds  to  a  vol- 
ume of  the  emanation  of  about  one  cubic  millimetre.  The  total 
heat  released  from  one  cubic  centimetre  of  the  emanation  and  its 
products  is  thus  about  ten  million  gram  calories. 

Now  in  the  union  of  hydrogen  with  oxygen  to  form  water 
more  heat  is  emitted,  weight  for  weight,  than  in  any  other  known 
chemical  reaction.  In  the  explosion  of  1  c.c.  of  hydrogen  with 
I  c.c.  of  oxygen  to  form  water,  3  gram  calories  of  heat  are 
emitted.  We  thus  see  that  the  transformation  of  the  emanation 
is  accompanied  by  nearly  four  million  times  as  much  heat  as  is 
given  out  by  the  union  of  an  equal  volume  of  hydrogen  with 
oxygen  to  form  water. 

If  we  assume  that  the  atom  of  the  emanation  has  200  times 
the  mass  of  the  hydrogen  atom,  it  can  readily  be  calculated 
that  one  pound  weight  of  the  emanation  would  emit  energy  at 
a  rate  corresponding  to  10,000  horsepower.  This  evolution  of 
energy  would  fall  off  exponentially,  but,  during  the  life  of  the 
emanation,  the  total  energy  released  would  correspond  to  about 
60,000  horsepower-days. 

These  figures  bring  out  in  a  striking  way  the  enormous  evolu- 
tion of  heat  accompanying  the  changes  in  the  emanation.  The 
amount  is  of  quite  a  different  order  of  magnitude  from  that 
absorbed  or  released  in  the  most  violent  chemical  reactions. 


92  RADIOACTIVE   TRANSFORMATIONS 

We  shall  see  later  (Chapter  X)  that  probably  every  radio- 
active product  which  expels  a  particles  emits  an  amount  of 
heat  of  the  same  order  of  magnitude  as  that  emitted  by  the 
emanation.  In  fact,  it  will  be  shown  that  this  evolution  of  heat 
is  a  necessary  accompaniment  of  their  radioactivity,  for  the  heat 
is  a  measure  of  the  kinetic  energy  of  the  a  particles  expelled 
from  the  emanation  and  its  products. 

DISCUSSION  OF  RESULTS 

We  may  now  briefly  summarize  the  properties  of  the  radium 
emanation  discussed  in  this  chapter.  (1)  The  emanation  is  a 
heavy  gas  which  does  not  combine  with  any  substances,  but 
appears  to  be  allied  in  general  properties  with  the  inert  group 
of  gases  of  which  helium  and  argon  are  the  best  known  ex- 
amples. (2)  It  diffuses  like  a  gas  of  high  molecular  weight 
and  obeys  Boyle's  law.  (3)  It  has  a  definite  spectrum  of 
bright  lines  analogous  to  the  spectra  of  the  inert  gases.  (4)  It  is 
condensed  from  a  mixture  of  gases  at  a  temperature  of  —150°  C. 
(5)  Unlike  ordinary  gases,  the  emanation  is  not  permanent,  but 
undergoes  transformation  according  to  an  exponential  law.  The 
volume  of  the  emanation  consequently  decreases  at  the  same 
rate  as  it  suffers  disintegration,  i.  e.,  its  volume  shrinks  to  half 
value  in  3.8  days.  The  transformation  of  the  emanation  is 
accompanied  by  the  expulsion  of  a  particles,  and  results  in  the 
appearance  of  a  new  series  of  non-gaseous  substances  deposited 
on  the  surface  of  bodies.  The  properties  of  the  active  deposit, 
and  the  changes  occurring  in  it,  will  be  discussed  in  detail  in 
the  next  chapter. 

The  emanation,  weight  for  weight,  is  about  one  hundred 
thousand  times  as  active  as  the  radium  from  which  it  is  derived. 
On  account  of  its  enormous  activity,  it  glows  in  the  dark  and 
causes  a  brilliant  phosphorescence  in  many  substances.  The 
rays  quickly  color  glass,  quartz,  and  other  bodies,  and  produce 
a  rapid  evolution  of  hydrogen  and  oxygen  in  a  water  solution. 
The  transformation  of  the  emanation  is  accompanied  by  an 
enormous  evolution  of  heat,  of  an  order  one  million  times 
greater  than  that  observed  in  any  chemical  reaction. 


THE   RADIUM   EMANATION  93 

We  have  seen  that  the  emanation  and  its  subsequent  prod- 
ucts are  responsible  for  three  quarters  of  the  activity  of 
radium  measured  by  the  a  rays.  The  emanation  itself  does 
not  emit  /3  or  7  rays,  but  these  arise  from  one  of  its  subsequent 
products.  Consequently  the  &  and  7  ray  activity  of  radium  is 
almost  completely  removed  by  depriving  it  of  its  emanation, 
provided  that  several  hours  have  been  allowed  to  elapse  in  order 
that  the  active  deposit  left  behind  with  the  radium  may  lose 
its  activity. 

The  emanation,  with  its  subsequent  products,  thus  contains 
the  concentrated  essence  of  the  radioactivity  of  radium.  A  tube 
containing  the  radium  emanation  has  all  the  radioactive  proper- 
ties of  radium  in  equilibrium.  It  emits  a,  ft,  and  7  rays,  evolves 
heat,  and  produces  luminosity  in  many  substances.  Radium 
itself,  freed  from  the  emanation  and  the  active  deposit,  emits 
only  a  rays.  Its  activity  and  heating  effect  under  such  condi- 
tions is  only  one  quarter  of  its  usual  value,  when  in  radioactive 
equilibrium. 

The  emanation  is  produced  from  radium  at  a  constant  rate, 
and  appears  to  be  a  direct  disintegration  product  of  the  radium 
atom.  Following  the  same  line  of  argument  previously  con- 
sidered, it  may  be  supposed  that  a  minute  fraction  of  the  total 
number  of  radium  atoms  explode  every  second,  each  violently 
ejecting  an  a  particle.  The  radium  atom,  minus  an  a  particle, 
becomes  the  new  substance  —  the  emanation.  The  atoms  of 
the  emanation  are  far  more  unstable  than  those  of  radium  itself, 
and  break  up  with  the  expulsion  of  a  particles  at  such  a  rate 
that  half  of  the  particles  are  transformed  in  3.8  days.  After 
the  expulsion  of  an  a  particle,  the  emanation  turns  into  the 
active  deposit. 

The  transformations,  so  far  considered,  and  the  rays  emitted, 
are  graphically  illustrated  below: 

„  a  particle 

Radium  a  particle 

EmanatioD 

*  Active  deposit 


94  RADIOACTIVE   TRANSFORMATIONS 

The  remarkable  differences  in  the  chemical  and  physical 
properties  of  a  disintegration  product  and  its  parent  substance 
are  strikingly  illustrated  by  the  comparison  of  radium  with  its 
emanation.  Radium  is  a  solid  substance  of  atomic  weight  225, 
closely  allied  in  ordinary  chemical  properties  with  barium.  It 
has  a  definite  well-marked  spectrum  analogous  in  many  respects 
to  the  spectra  of  the  rare  earths.  It  is  non-volatile  at  ordinary 
temperatures,  and  apart  from  its  radioactivity  has  all  the  proper- 
ties of  a  new  element  very  analogous  to  barium.  On  the  other 
hand,  the  emanation  is  an  inert  gas  which  cannot  be  made  to 
combine  with  any  substance.  Its  spectrum  of  bright  lines  is 
similar  in  general  appearance  to  the  spectra  of  the  helium-argon 
family  of  gases.  It  is  condensed  at  a  temperature  of  —150°  C. 
Apart  from  its  radioactivity,  the  properties  of  the  emanation 
are  thus  entirely  different  from  those  of  the  parent  radium,  and, 
if  we  had  no  proof  of  its  production  by  radium,  there  would  be 
no  reason  to  believe  they  were  in  any  way  connected  with  each 
other. 


CHAPTER   IV 
TRANSFORMATION   OF   THE   ACTIVE   DEPOSIT   OF   RADIUM 

IN  the  previous  chapter  attention  has  been  drawn  to  the  fact 
that  all  bodies  surrounded  by  the  radium  emanation  become 
coated  with  an  invisible  active  deposit,  possessing  physical  and 
chemical  properties  which  sharply  distinguish  it  from  the 
emanation.  This  property  of  radium  of  "  exciting  "  or  "  in- 
ducing "  activity  in  neighboring  bodies  was  first  observed  by 
P.  Curie,1  and  has  in  recent  years  been  the  subject  of  a  number 
of  investigations. 

In  this  chapter,  the  transformations  taking  place  in  this 
active  deposit  will  be  discussed,  and  it  will  be  shown  that,  in 
general,  the  deposit  consists  of  a  mixture  of  three  distinct  sub- 
stances called  radium  A,  B,  and  C.  Radium  A  arises  directly 
from  the  transformation  of  the  emanation,  radium  B  arises  from 
radium  A,  and  radium  C  from  radium  B. 

The  three  products  are  thus  derived  by  the  successive  dis- 
integration of  the  emanation.  The  analysis  of  these  stages  is 
somewhat  more  difficult  than  in  the  case  of  two  changes  already 
considered  for  thorium,  but  can  be  attacked  by  the  same  general 
methods. 

The  active  deposit  of  radium  is  analogous  in  many  respects 
to  the  corresponding  deposit  produced  by  the  thorium  emana- 
tion. It  is  a  material  substance,  which,  in  the  absence  of  an 
electric  field,  is  deposited  from  the  gas  on  the  surface  of  all 
bodies  in  contact  with  the  emanation.  In  a  strong  electric  field 
it  is  mostly  concentrated  on  the  negative  electrode.  In  this 
respect  it  behaves  similarly  to  the  active  deposit  of  thorium. 
The  active  matter  can  be  partly  removed  from  a  platinum 
wire  -by  solution  in  hydrochloric  acid,  and  remains  behind 

1  M.  and  Mme.  Curie,  Comptes  rendus,  cxxix,  p.  714  (1899). 


96  RADIOACTIVE   TRANSFORMATIONS 

on  the  dish  when  the  acid  is  driven  off  by  heat.  By  using 
the  emanation  from  about  10  milligrams  of  radium  bromide,  a 
wire  can  be  made  intensely  active.  It  causes  brilliant  fluores- 
cence on  a  screen  of  willemite  or  zinc  sulphide  brought  near  it. 
The  deposit  is  entirely  confined  to  the  surface  of  a  conductor. 
If  a  strongly  active  wire  is  drawn  across  a  screen  of  willemite 
or  other  substance  which  lights  up  under  the  action  of  the 
rays,  a  bright  luminous  trail  is  left  behind.  This  is  due  to  the 
removal  of  some  of  the  deposit  by  the  particles  of  the  screen 
over  which  it  has  been  rubbed.  The  luminosity  left  behind 
gradually  decreases,  and  is  very  small  after  3  hours.  The  re- 
moval of  the  active  deposit  by  rubbing  is  also  easily  shown  by 
bringing  near  an  electroscope  a  piece  of  cloth  which  has  been 
drawn  over  the  active  wire.  The  electroscope  is  discharged 
almost  instantly,  and  this  discharging  property  persists,  but  with 
diminishing  amount,  for  several  hours. 

In  the  case  of  a  short-lived  emanation  like  that  of  thorium, 
the  excited  activity,  in  the  absence  of  an  electric  field,  is 
greatest  on  bodies  placed  near  the  emanating  thorium  com- 
pound. This  result  is  to  be  expected,  since  the  emanation  is 
decomposed  before  it  has  time  to  diffuse  far  from  its  source. 
On  the  other  hand,  in  a  similar  enclosure  containing  radium  as 
a  source  of  emanation,  the  excited  activity  is  produced  on  all 
bodies  placed  in  the  vessel.  In  this  case  the  life  of  the  emana- 
tion is  long  compared  with  the  time  taken  for  the  emanation  to 
be  distributed  by  the  processes  of  diffusion  to  all  parts  of  the 
enclosure. 

Bodies  which  are  completely  screened  from  the  direct  radia- 
tion of  the  radium  become  active.  This  is  clearly  brought  out 
in  an  experiment  made  by  P.  Curie,  which  is  shown  in  Fig.  23. 

A  small  open  vessel,  a,  contained  a  radium  solution,  giving 
off  emanation  at  a  constant  rate.  This  was  placed  in  a  closed 
vessel  in  which  plates  A,  B,  C,  D,  E  were  fixed  in  various 
positions.  After  a  day's  exposure,  all  the  plates  on  removal 
were  found  to  be  active,  even  that  in  the  position  D,  com- 
pletely shielded  from  the  direct  radiation  of  the  radium  by  a 
lead  block  P. 


TRANSFORMATION   OF  RADIUM 


97 


The  amount  of  activity  per  unit  area  on  a  plate  in  a  given 
position  is  independent  of  the  material  of  the  plate.  A  plate 
of  mica  becomes  just  as  active  as  one  of  metal.  The  amount 
of  excited  activity  on  a  given  area  depends  to  some  extent  on 
the  free  space  in  the  neighborhood.  The  lower  surface  of  the 
plate  A,  for  example,  would  be  less  active  than  the  upper  sur- 
face, since  the  active  deposit  on  the  lower  side  arises  mainly 
from  the  small  volume  of  emanation  between  it  and  the  en- 
closure, while  the  upper  surface  of  the  plate  gains  the  active 
deposit  generated  in  a  much  larger  volume. 

The  emanation  from 
several  milligrams  of 
radium  bromide  causes 
so  great  an  activity  on 
a  wire  or  metal  plate 
exposed  in  its  presence, 
that  the  ionization  cur- 
rent produced  by  it  can 
be  readily  measured  by  a 
sensitive  galvanometer. 
With  such  intensely 
active  plates,  a  large 
voltage  is  required  to 
produce  a  saturation 
current  through  the  gas 
unless  the  plates  of  the 
testing  vessel  are  placed 
close  together. 

We  shall  first  consider  the  evidence  in  support  of  the  view 
that  the  active  deposit  is  a  disintegration  product  of  the 
radium  emanation.  If  some  radium  emanation  is  introduced 
into  a  cylindrical  testing  vessel  such  as  is  shown  in  Fig.  10, 
and  the  ends  closed,  the  activity,  measured  by  the  saturation 
current  through  the  gas,  increases  with  the  time  for  several 
hours,  generally  reaching  about  twice  the  value  observed  at  the 
moment  of  introduction  of  the  emanation.  The  comparative 
increase,  however,  varies  to  some  extent  with  the  dimensions 

7 


FIG.  23. 

Distribution  of  excited  activity  on  bodies  in  the 
presence  of  the  radium  emanations. 


98  RADIOACTIVE   TRANSFORMATIONS 

of  the  testing  vessel,  on  account  of  the  difference  in  penetrating 
power  of  the  a  rays  emitted  by  the  various  products. 

When  the  emanation  is  blown  out,  the  active  deposit  is  left 
behind,  and  loses  the  greater  part  of  its  activity  in  a  few  hours. 
This  property  of  producing  an  active  deposit  is  not  shown  by 
radium  which  has  been  freed  from  emanation,  but  belongs 
to  the  emanation  alone.  The  excited  activity  produced  in 
bodies  is  directly  proportional  to  the  amount  of  emanation 
present,  no  matter  how  old  the  emanation  may  be.  For  ex- 
ample, if  the  emanation,  which  still  remains  after  being  stored 
in  a  gas  holder  for  a  month,  is  passed  into  a  testing  vessel, 
excited  activity  is  still  produced,  and  in  an  amount  which  bears 
the  same  ratio  to  the  activity  of  the  emanation  present  as  for  a 
new  sample  of  emanation  tested  immediately  after  its  release 
from  radium. 

The  constancy  of  this  ratio  between  the  amount  of  the  ema- 
nation present  and  the  amount  of  active  deposit  produced  is 
at  once  explained  if  the  emanation  is  the  parent  of  the  active 
deposit.  For  example,  suppose  that  a  body  is  exposed  to  a 
constant  supply  of  emanation.  The  activity  imparted  to  the  body 
reaches  a  steady  limit  after  about  5  hours.  There  is,  then,  a  state 
of  equilibrium  between  the  active  deposit  and  the  emanation. 
Under  such  conditions,  the  number  of  atoms  of  radium  A  which 
break  up  per  second  must  equal  the  number  of  new  atoms 
of  radium  A  supplied  per  second  by  the  decomposition  of  the 
emanation.  This  in  turn  is  equal  to  the  number  of  atoms  of 
emanation  which  break  up  per  second.  A  similar  result  also 
holds  true  for  radium  B  and  C.  Since  the  number  of  atoms  of 
any  individual  product  which  break  up  per  second  is  always 
proportional  to  the  total  number  present,  it  is  seen  that  the 
equilibrium  number  of  atoms  of  radium  A  must  always  be  pro- 
portional to  the  number  of  atoms  of  emanation.  If  X  is  the 
constant  of  decay  of  the  emanation  and  X^,  XB,  Xc,  the  constants 
for  radium  A,  B,  and  C,  respectively,  then  the  equilibrium 
amounts  NA,  NB,  NC,  respectively,  of  the  three  products  are 
given  by  the  equations 

*A&A  =  *BNB  =  Ac^c=  X.2V, 


TRANSFORMATION   OF   RADIUM  99 

where  N  is  the  total  number  of  atoms  of  emanation  present. 
When  a  state  of  equilibrium  has  been  reached,  the  number  of 
atoms  of  each  product  present  will  be  different,  being  directly 
proportional  to  the  period  of  each  product.  A  rapidly  changing 
substance  will  consequently  be  present  in  less  amount  than  a 
slowly  changing  one. 

After  introducing  the  emanation  into  a  closed  vessel,  its 
amount,  as  we  have  seen,  decreases  exponentially.  Since,  how- 
ever, the  periods  of  the  products  of  the  active  deposit  are  small 
compared  with  that  of  the  emanation  itself,  the  amount  of  the 
active  deposit  will,  after  a  few  hours,  nearly  reach  an  equi- 
librium value,  and  will  then  decrease  pari  passu  with  the 
emanation. 

The  excited  activity  will  thus  fall  off  at  the  same  rate  as  the 
activity  of  the  emanation.  This  proportion  has  been  utilized, 
as  we  have  already  seen,  by  Curie  and  Danne,  to  determine  the 
constant  of  decay  of  the  emanation  by  measurement  of  the  y3 
and  7  rays  which  escape  from  the  active  deposit  through  the 
walls  of  a  closed  vessel  containing  the  emanation. 

ACTIVITY  CURVES  OF  THE  ACTIVE  DEPOSIT 

We  shall  now  consider  in  detail  the  variation  with  the  time 
of  the  activity  of  this  deposit  under  different  conditions.  The 
experimental  results  are  at  first  sight  very  complicated,  for  the 
activity  curves  not  only  vary  remarkably  with  the  time  of  ex- 
posure to  the  emanation,  but  also  depend  on  whether  the  a,  /?, 
or  7  rays  are  used  as  a  means  of  measurement.  It  is  thus  very 
necessary  in  each  case  to  specify  carefully,  not  only  the  time 
of  exposure  to  the  emanation,  but  also  the  type  of  rays  used  for 
measurement. 

The  decay  curves  of  the  active  deposit  are  independent  of 
the  nature  and  size  of  the  body  that  has  been  made  active  and 
of  the  amount  of  emanation  to  which  it  has  been  exposed.  If 
a  wire  is  to  be  made  active  the  arrangement  shown  in  Fig.  24 
is  very  suitable. 

A  solution  of  radium  is  placed  in  a  vessel  closed  by  a  rubber 
stopper.  The  emanation  collects  in  the  air  space  above  the 


100 


RADIOACTIVE   TRANSFORMATIONS 


w 


solution.  The  thin  wire,  W,  to  be  made  active  is  fixed  into 
a  fine  hole  bored  in  the  end  of  a  central  rod.  This  rod  slips 
freely  through  an  ebonite  cork  fixed  in  a  brass  tube,  B.  A 
platinum  wire  P  passes  through  the  rubber  cork  and  dips  into 
the  solution. 

The  platinum  wire  is  in  metallic  connection  with  the  brass 
tube.  The  central  rod  is  connected  with  the  negative  pole  of 

a  battery  of  300  or  400  volts,  and 
the  platinum  wire  with  the  positive. 
Under  such  conditions,  the  moist 
walls  of  the  glass  vessel,  the  solu- 
tion, and  the  tube  B,  are  charged 
positively,  and  the  wire  W  is  the 
only  negatively  charged  body  in 
the  presence  of  the  emanation. 
The  active  deposit  is  consequently 
concentrated  upon  it,  and,  in  the 
presence  of  a  large  amount  of  ema- 
nation, the  activity  of  the  wire 
becomes  very  great. 

After  introducing  the  wire,  a 
little  hard  wax  is  run  round  the 
top  of  the  rod,  to  prevent  the  es- 
cape of  the  emanation.  When 
the  wire  has  been  exposed  for  the 
FlG-  24-  interval  required,  the  rod  is  re- 

Arrangement  for  concentrating  the   moved  and  the  active  wire  released. 

active  deposit  derived  from  the    gince    ^    fine   wire    ig    of   smaller 
radium    emanation  on  a    small 

negatively  charged  wire.  diameter  than  the  rod,  the  wire 

need  not  touch  the  side  during 
removal,  so  that  none  of  the  active  deposit  is  rubbed  off. 

In  order  4o  test  the  variation  of  the  a  ray  activity  of  this  wire 
with  time,  it  is  attached  to  the  end  of  a  brass  rod  forming  the 
central  electrode  of  a  testing  vessel  such  as  is  shown  in  Fig.  10. 

If  a  greater  surface  is  to  be  made  active,  a  sheet  of  metal  is 
placed  in  a  glass  tube  closed  at  both  ends.  The  emanation 
is  introduced  after  first  exhausting  the  vessel,  and  the  active 


TRANSFORMATION   OF  RADIUM 


101 


matter  is  then  deposited  upon  the  metal  by  the  process  of  dif- 
fusion. After  removal,  the  activity  of  the  plate  is  tested  elec- 
trically, using  a  parallel  plate  apparatus  similar  to  that  described 
in  Fig.  9. 

a  RAY  CURVES 

We  shall  first  consider  the  decay  of  activity,  measured  by 
the  a  rays,  for  a  body  exposed  for  a  short  time  in  the  presence 
of  the  emanation.  The  time  of  exposure  —  not  more  than  one 


100 


Decay 


of   Excited 
measured 


activitjy 
by    ex 


of  Rad 


um 


rays. 


Sho 


Exposure    (i  mirute) 


ID 


2O  30  40  5O  OO  7O 

Time    in   Minutes. 
FIG.  25. 

minute  —  is  supposed  to  be  short  compared  with  the  period  of 
the  changes  hi  the  active  matter.  The  results  are  shown  in 
Fig.  25,  Curve  BB,  the  maximum  activity  immediately  after 
removal  being  taken  as  100. 

The  activity  at  first  decreases  very  nearly  according  to  an 
exponential  law,  falling  off  to  half  value  in  about  3  minutes. 
After  20  minutes,  the  activity  is  less  than  10  per  cent  of  the 
initial  value,  and  remains  nearly  constant  for  a  further  20 
minutes,  and  then  gradually  decays.  After  several  hours  the 


102  RADIOACTIVE   TRANSFORMATIONS 

activity  again  decreases  nearly  exponentially  with  a  period  of 
28  minutes. 

In  the  same  figure  (Curve  AA)  is  shown  the  a  ray  decay 
curve  for  a  long  exposure.  The  time  of  exposure  in  this  case 
(about  5  hours  will  suffice)  is  supposed  to  be  sufficient  to  allow 
the  active  deposit  and  the  emanation  to  have  very  nearly 
reached  a  stage  of  radioactive  equilibrium.  There  is  initially 
a  rapid  decay  with  a  3  minute  period,  and  then  a  gradual  de- 
crease at  a  slower  rate  than  is  given  by  an  exponential  law. 
After  about  5  hours  the  decay  curve  is  nearly  exponential,  fall- 
ing to  half  value  in  about  28  minutes. 

The  initial  rapid  change  with  a  3  minute  period  is  due  to 
the  product  radium  A.  The  final  exponential  decay  with  a 
28  minute  period  shows  that  another  product,  having  a  28 
minute  period,  is  also  present.  Before  discussing  the  explana- 
tion of  the  intermediate  portion  of  the  two  curves  the  activity 
curves  measured  by  the  /3  and  7  rays  will  first  be  considered. 

/3  RAY  CURVES 

In  order  to  determine  the  0  ray  curves,  an  electroscope  was 
used.  The  active  plate  or  wire  was  placed  under  the  base  of 
the  electroscope,  which  was  covered  with  a  sheet  of  aluminium 
of  sufficient  thickness  to  absorb  all  the  a  rays.  The  discharge 
produced  in  the  electroscope  is  then  due  to  the  /3  and  y  rays 
together,  the  effect  of  the  former  preponderating.  The  curve 
in  Fig.  26  shows  the  variation  of  the  /?  ray  activity  with  time, 
for  a  wire  which  had  been  exposed  for  one  minute  in  the 
presence  of  a  large  amount  of  emanation.  It  will  at  once  be 
observed  that  the  curve  is  entirely  different  in  character  from 
the  corresponding  a  ray  curve  shown  in  Fig.  25.  The  ft  ray 
activity  is  small  at  first,  but  increases  with  time,  reaching  a 
maximum  after  about  35  minutes.  Several  hours  later  it  decays 
nearly  exponentially  with  a  period  of  28  minutes. 

The  /3  ray  curve  for  a  long  exposure  to  the  emanation  is 
shown  in  Fig.  27. 

The  curve  is  very  different  in  shape  from  the  short  exposure 
curve.  The  activity  does  not  increase  initially,  but  falls,  first 


TRANSFORMATION   OF   RADIUM 


103 


slowly  and  then  more  quickly.     Finally,  as  in  the  other  cases, 
it  decreases  exponentially  with  a  period  of  28  minutes. 

7  RAY  CURVES 

The  curves  for  a  short  and  long  exposure  measured  by  the 
7  rays  alone  are  identical  with  those  obtained  for  the  ft  and 
7  rays  together.  The  measurements  were  made  with  an  elec- 
troscope, the  rays  passing  through  about  1  cm.  of  lead  before 


100 


8C 


60 


Short 


j  Curvj 
Expof  ure. 


£40 


20 


O  15  30  45  60  75  90  105  120 

Time    in    Minutes. 

FIG.  26. 

Variation  of  the  activity,  measured  by  the  #  rays,  of  a  body  exposed  for 
a  short  interval  to  the  radium  emanation. 

entering  the  electroscope.     This  insures  that  the  ft  as  well  as 
the  a  rays  are  cut  off  completely.      :• 

The  identity  of  the  ft  and  7  ray  curves  shows  that  the  two 
kinds  of  rays  always  occur  in  the  same  proportion.  This  rela- 
tion is  a  strong  argument  in  favor  of  the  view  that  the  7  rays 
are  a  type  of  X-rays,  which  are  set  up  at  the  moment  of  the  ex- 
pulsion of  the  ft  particle  from  radioactive  matter.  This  ratio 
between  the  intensities  of  the  two  kinds  of  rays  has  been  shown 
to  hold  in  every  case  so  far  examined,  and  suggests  that  the 


104 


RADIOACTIVE  TRANSFORMATIONS 


7  rays  bear  the  same  relation  to  the  (S  rays  that  the  X-rays  bear 
to  the  cathode  rays. 

THEORY  OF  SUCCESSIVE  CHANGES  IN  RADIUM 

We  shall  show  later  that  the  peculiarities  of  the  decay  curves 
of  the  active  deposit  of  radium  for  any  time  of  exposure,  whether 


100 


80 


60 


Dec 


ay  of  Excited  A 


of 


tadium. 


fcivlfcy 


o  20  40  60  So          100         120 

Time  in  Minutes. 

FIG.  27. 

Variation  of  the  activity,  measured  by  the  £  or  y  rays,  of  a  body  exposed 
for  a  long  interval  to  the  radium  emanation. 

the  activity  is  measured  by  the  a,  /3,  or  7  rays,  can  be  satisfac- 
torily explained  on  the  following  assumptions :  — 

(1)  That  the  emanation  is  transformed  into  a  product  called 
radium  A,   which  emits  only  a  rays,  and  has  a  period   of 
minutes. 

(2)  That  radium  A  is  transformed  into  radium  B,  which  has 
a  period  of  28  minutes,  and  is  transformed  without  the  emis- 
sion of  a,  /3,  or  7  rays.     In  other  words,  radium  B  is  a  rayl< 
product. 


TRANSFORMATION   OF  RADIUM  105 

(3)  Radium  B  is  transformed  into  radium  C,  which  has  a 
period  of  21  minutes,  and  emits  during  its  transformation  a,  /:?, 
and  7  rays. 

We  thus  have  to  deal  with  the  problem  of  three  successive 
changes.  Since,  however,  the  first  product,  radium  A,  is 
rapidly  transformed  with  a  3  minute  period,  the  amount  of  it 
remaining,  for  example,  21  minutes  after  removal,  is  only  1/128 
of  the  initial  amount. 

For  simplicity,  therefore,  in  the  discussion  of  the  activity 
curves  measured  by  the  /3  rays,  we  shall  for  the  moment  disre- 
gard the  first  rapid  change,  and  suppose  that  the  emanation  is 
transformed  directly  into  radium  B.  As  a  matter  of  fact,  it  is 
found  that  the  experiments  agree  better  with  theory  if  the  first 
transformation  is  disregarded  altogether.  A  possible  explana- 
tion of  this  peculiarity  in  the  curves  will  be  considered  later. 

In  the  discussion  of  the  activity  curves  for  the  active  deposit 
of  thorium,  it  has  been  shown  that  the  experimental  curve  for 
a  short  exposure  may  be  satisfactorily  explained  if  the  emana- 
tion is  supposed  to  change  into  the  rayless  product,  thorium  A, 
which  has  a  period  of  11  hours.  This  in  turn  is  transformed 
into  thorium  B,  which  emits  a,  /3,  and  7  rays,  and  has  a  period 
of  about  1  hour.  These  results  deduced  from  analysis  of  the 
activity  curves  have  been  completely  substantiated  by  experi- 
ments in  which  the  products  thorium  A  and  B  have  been  sepa- 
rated from  each  other  by  various  physical  and  chemical  methods. 

The  case  of  radium  is  very  analogous,  for,  disregarding  the 
first  3  minute  change,  the  product  radium  B  emits  no  rays, 
but  changes  into  radium  C,  which  emits  a,  /:?,  and  7  rays. 

We  shall  now  consider  the  theory  of  two  successive  changes 
of  the  character  explained  above. 

Let  Xj,  X2  be  the  constants  of  change  of  the  products  radium  B 
and  C  respectively. 

Let  P  and  Q  be  the  number  of  atoms  of  B  and  C  respectively 
present  at  any  time  after  removal  from  the  emanation. 

Two  general  cases  will  first  be  considered,  corresponding  to 
a  short  and  long  exposure  of  a  body  to  the  radium  emanation. 


106  RADIOACTIVE   TRANSFORMATIONS 

CASE  OF  A  SHORT  EXPOSURE 

The  matter  initially  deposited  is  supposed  to  be  all  of  one 
kind,  radium  B.  Let  n  be  the  number  of  particles  of  B  that 
have  been  deposited.  The  number,  P,  of  these  remaining  at 
any  time  £,  after  removal,  is  given  by 

P  =  n  e-M. 

We  have  shown  on  page  50  that  the  rate  of  change  of  the 
number  Q  of  atoms  of  C  existing  at  any  time,  £,  after  removal, 
is  given  by 

dQ-\p     AO 

—  -X,f  -\2Q  (1) 

=  X1n  er-W  —  \tQ. 

The  solution  of  this  equation  (see  page  51)  shows  that  Q  is 
given  by 

Q  =  -±* 


The  number  of  atoms  of  P  and  Q  existing  at  any  time  after 
removal  is  shown  in  Fig.  28.  The  initial  number  of  atoms  of 
B  deposited  is  supposed  to  be  100.  The  exponential  curve,  BB, 
expresses  the  amount  of  B  remaining  unchanged  at  any  time. 
The  curve  CC  shows  the  number  of  atoms  of  radium  C  existing 
at  any  time.  The  periods  of  the  changes  of  B  and  C  are  about 
28  and  21  minutes  respectively,  so  that 

A!  =  4.13  X  10~4  (sec.)"1,        A,,  =  5.38  X  10-4  (sec.)-1. 

The  amount  of  radium  C,  initially  zero,  increases  to  a  maximum 
in  about  35  minutes,  and  then  diminishes,  and  about  5  hours  later 
decays  exponentially,  with  a  period  of  28  minutes.  The  amount 
of  C  will  thus  decrease,  not  according  to  its  own  period,  but 
according  to  the  longer  period  of  the  rayless  product.  This  is 
easily  shown  from  the  equation  for  §,  which  may  be  expressed 
in  the  form 


After  7  hours,  e-(^-W  =  .043, 


TRANSFORMATION   OF   RADIUM 


107 


and  is  thus  almost  negligible.     Q  then  varies  very  nearly  as 
e~^*,  i.  e.,  according  to  the  period  of  the  rayless  product. 

Since  B  does  not  emit  rays  and  C  does,  the  value  of  Q  at  any 
time  is  proportional  to  the  activity  of  the  mixture  of  products 
B  and  C. 


too 


BO 

t> 

8 

5- 

1 


20 


20 


/CO 


/SO 


40  00  8O 

T/MC  tfi  MMUTZ9. 

FIG.  28. 

Theoretical  curves  showing  the  variation  of  the  number  of  atoms  of  radium  B 
and  radium  C  when  the  matter  present  initially  consists  only  of  radium  B. 

The  curve  CC  should  thus  be  identical  in  form  with  the 
curves  for  a  short  exposure  measured  by  the  /B  or  7  rays,  and 
within  the  limits  of  experimental  error  this  is  found  to  be  so. 

CASE  OF  A  LONG  EXPOSURE 

Suppose  that  P0  and  Q0  are  the  equilibrium  numbers  of  atoms 
of  B  and  C  present  after  a  long  exposure  to  the  emanation. 
Under  such  conditions 

\l  PQ  =  X2   QQ  =•  q, 


108  RADIOACTIVE   TRANSFORMATIONS 

where  q  is  the  number  of  atoms  of  emanation  breaking  up  per 
second. 

The  value  of  P,  the  number  of  atoms  of  radium  B  present 
at  any  time  t  after  removal  from  the  emanation  is  given  by 

fo  /  P  = 


The  value  of  Q  is  given  by  equation  (1)  as  before.    The  solu- 
tion of  this  equation  is  of  the  form 


Q  =  a  e-M  +  b 
By  substitution  in  equation  (1)  it  is  seen  that 

9 


a  = 


Since  initially  when  t  =  0,  Q  =  Q0  =  — , 

AJJ 

we  have  a  +  b  =  —• ; 

A2 

thus  h  -        ~"  g  Ai 

' 


and  Q  =  — * —  (e-M «-*«*).  (2) 

A2  —  AI  A2 

The  variation  of  the  amount  of  radium  B  with  time  after 
a  long  exposure  is  shown  in  Fig.  29,  the  number  of  atoms  of 
B  initially  present  being  100.  The  number  of  atoms  of  radium 

C  present  initially  is  jr-ZV 

The  curve,  CC,  expressing  the  number  of  atoms  of  C  presen 
at  any  time  thus  begins  at  a  point  whose  ordinate  is  77  insteac 
of  100. 

Since  the  ft  or  7  ray  activity  of  C  is  proportional  at  any  time 
to  the  value  of  $,  the  curve  showing  the  variation  of  radium  C 
with  time  should  be  of  the  same  form  as  the  activity  curve  in 
Fig.  27  for  a  long  exposure,  as  measured  by  the  {S  and  7  rays 
This  is  the  case,  for  the  theoretical  and  observed  curves  agree 
within  the,  limit  of  experimental  error.  This  is  shown  in  the 
following  table: 


TRANSFORMATION   OF   RADIUM 


189 


DECAY  or  ACTIVITY  MEASURED  BY  THE  $  RAYS  FOR  A  LONG  EXPOSURE 
TO  THE  EMANATION. 


Time  in  minutes 
after  removal 
from  emanation. 

Observed 
activity. 

Theoretical 
activity. 

0 

100 

100 

10 

97.1 

96.8 

20 

88. 

89.4 

30 

77., 

78.6 

40 

67. 

69.2 

50 

57.< 

59.9 

60 

48.: 

49.2 

80 

33. 

34.2 

100 

22.i 

22.7 

120 

14.5 

14.9 

/oo 


60 


40 


2O 


20 


40 


60  $O 

IN    MINUTES., 


/OO 


120, 


FIG.  29. 


Theoretical  curves  showing  the  number  of  atoms  of  radium  B  and  radium  C 
existing  at  any  time,  when  the  matter  initially  consists  of  radium  B  and  C  in 
radioactive  equilibrium. 


110 


RADIOACTIVE   TRANSFORMATIONS 


The  fact  that  the  long  exposure  curve  shown  in  Fig.  27  re- 
sults from  two  successive  products,  the  first  of  which  does  not 
emit  rays  at  all,  can  readily  be  shown  by  graphical  analysis. 

Immediately  after  removal  of  the  active  body  from  the  emana- 
tion, the  active  deposit  consists  of  B  and  C  in  equilibrium. 
The  /3  ray  activity  observed  is  due  entirely  to  C,  and,  leaving 


/oo 


120 


FIG.  30. 


Analysis  of  the  ft  ray  curve  for  a  long  exposure  to  the  emanation,  in  order  to 
show  that  it  results  from  the  presence  of  two  products,  the  first  of  which  is 
rayless. 

out  of  account  for  the  moment  the  fresh  supply  of  C  froi 
the  disintegration  of  B,  the  amount  of  C  must,  if  left  to  itself, 
diminish  exponentially  following  the  period  of  C,  i.  e.,  the 
activity  will  fall  to  half  value  in  21  minutes.  This  decay 
curve  CC  is  shown  in  Fig.  30.  Now  the  difference  at  any 
time  between  the  ordinates  of  the  observed  curve  B  +  C,  an< 


TRANSFORMATION   OF   RADIUM  111 

the  theoretical  curve,  CC,  must  be  due  to  the  activity  of  C, 
supplied  by  the  breaking  up  of  B.  This  difference  curve,  BB 
(see  Fig.  30),  should  be  identical  in  shape  with  the  (S  ray  curve 
of  the  active  deposit  for  a  short  exposure.  This  must  evidently 
be  the  case,  since  this  curve  gives  the  activity  arising  from  the 
transformation  of  B  alone,  all  the  matter  present  initially  being 
of  the  kind  B,  which  changes  into  C. 

By  comparison  of  the  curve,  BB,  with  the  short  exposure 
curve  shown  in  Fig.  27,  this  identity  is  seen  to  hold.  The 
activity  rises  from  zero,  and  reaches  a  maximum  after  35 
minutes  and  then  decays. 

It  is  of  interest  to  observe  that  the  empirical  equation  of  the 
decay  curve  of  the  /3  ray  activity  for  a  long  exposure  to  the 
emanation  was  obtained  before  the  theoretical  explanation  was 
advanced.  Curie  and  Danne  1  found  that  the  activity  It  at  any 
time  could  be  expressed  by  an  equation  of  the  form 


where      \^  =  4.13  X  1Q-4  (sec)-1    and  X  2  =  5.38  X  10-*  (sec)-1, 

and  a  =  4.20  is  a  numerical  constant.  The  constant  \  was  de- 
termined from  the  observed  fact  that  the  activity  several  hours 
after  removal  from  the  emanation  decayed  exponentially  with 
a  period  of  28  minutes.  The  values  of  a  and  X2  were  deter- 
mined so  as  to  fit  the  curve.  Now  this  equation  is  identical  in 
form  with  the  theoretical  equation  for  the  activity  when  the  first 
change  is  rayless  with  a  period  of  28  minutes,  and  the  second 
change,  which  has  a  period  of  21  minutes,  gives  out  rays. 
This  is  easily  seen  to  be  the  case.  From  equation  (2),  the  amount 
Q  of  radium  C,  existing  at  any  time  £,  is  given  by 


Ag  —  AI  A«j 

Initially  Q  =  Q0  =  Xz  q. 

1  Curie  and  Danne  :  Comptes  rendus,  cxxxvi,  p.  364  (1903). 


112  RADIOACTIVE   TRANSFORMATIONS 

Since  the  activity  at  any  time  is  proportional  to  the  amount 
of  C  present,  i.  e.,  to  the  value  of  §, 


It  =  Q_ 

-*0  VO 


On  substituting  the  values  of  \v   A2,  which  correspond  to 
periods  of  28  and  21  minutes  respectively, 


Xz      =  4.3    and    — ^-_  =  3.3. 


A2  -A!  A2  -  A, 

Thus  the  theoretical  equation  not  only  agrees  in  form  with 
that  deduced  from  observation,  but  the  values  of  the  constants 
are  very  concordant. 

Such  a  relation  between  theory  and  experiment  would  be 
widely  departed  from  if  B  as  well  as  C  gave  out  /3  rays. 

ANALYSIS  OF  THE  a  RAY  CURVES  FOB  A  LONG  EXPOSURE 

We  are  now  in  a  position  to  analyze  the  a  ray  activity  curve 
for  a  long  exposure  into  its  three  components.  In  this  case 
we  must  take  into  account  the  first  product,  radium  A,  which 
^emits  a  rays.  The  observed  a  ray  curve  is  shown  in  Fig.  31, 
<iurve  A  +  B  +  C.  This  curve  was  obtained  by  means  of  a 
galvanometer.  A  piece  of  platinum  foil  was  placed  for  several 
days  in  a  glass  vessel  containing  a  large  supply  of  radium 
emanation.  The  foil  was  then  rapidly  removed,  placed  on  the 
lower  plate  of  a  testing  vessel,  and  a  saturating  voltage  applied. 
The  variation  of  the  a  ray  activity  was  measured  by  means  of 
a  high  resistance  galvanometer.  The  initial  value  of  the  cur- 
rent at  the  instant  of  removal  was  deduced  by  continuing  the 
curve  backwards  to  meet  the  vertical  axis. 

The  variation  of  activity  with  time  is  shown  in  the  following 
.table: 


TRANSFORMATION   OF  RADIUM 


113 


Time  in 
minutes. 

Activity. 

Time  in 
minutes. 

Activity. 

0 

100 

30 

40.4 

2 

80 

40 

35.6 

4 

69.5 

50 

30.4 

6 

62.4 

60 

25.4 

8 

57.6 

80 

17.4 

10 

62.0 

100 

11.6 

15 

48.4 

120 

7.6 

20 

45.4 

The  activity  due  to  A  alone  has  almost  vanished  after  20 
minutes.  At  the  20  minutes  point  the  curve  B  -f  C  is  produced 
backwards,  meeting  the  axis  at  L.  It  cuts  it  at  about  50.  The 
difference  between  the  ordinates  of  the  curves  A  +  B  +  C  and 
LL  represents  the  activity  due  to  radium  A,  and  is  shown  by 


IQO 


60 


o* 


20 


60  eo 

IN  MINUTES. 


IQO 


/no 


FIG.  31. 


Analysis  of  the  a  ray  curve  for  a  long  exposure  to  the  radium  emanation,  show- 
ing that  it  results  from  the  presence  of  three  products,  A,  B,  and  C.  Radium 
A  and  C  emit  a  rays,  while  radium  B  does  not. 

8 


114  RADIOACTIVE   TRANSFORMATIONS 

the  curve  AA,  in  the  figure.  The  curve  A  is  exponential  with 
a  period  of  3  minutes.  The  curve  LL,  B  +  C,  is  identical  in 
form  over  its  whole  range  with  the  activity  curve,  for  a  long 
exposure  (see  Fig.  27),  measured  by  the  ft  rays.  We  may 
conclude  from  this  result  that  radium  B  emits  no  a  rays.  We 
know  already  that  it  emits  no  /8  rays ;  hence  radium  B  must  be 
a  rayless  product. 

The  curve  LL,  B  +  C,  can  be  analyzed  into  its  two  compo- 
nents exactly  in  the  same  way  as  the  corresponding  /3  ray  curve. 
The  curve  CC  represents  the  variation  in  the  activity  of  the 
radium  C  which  existed  at  the  moment  of  removal  from  the 
emanation.  The  curve  BB  represents  the  activity  due  to  C,  sup- 
plied by  the  change  of  B  into  C.  This  curve,  BB,  is  identical 
in  form  with  the  /3  ray  curve  for  a  short  exposure  (see  Fig.  26). 

We  may  conclude  from  this  analysis  that  the  active  deposit 
consists  of  three  products,  radium  A,  B,  and  C,  which  have 
the  following  peculiarities :  — 

Radium  A  emits  only  a  rays,  and  is  half  transformed  in 
3  minutes. 

Radium  B  is  a  rayless  product,  and  is  half  transformed  in  28 
minutes. 

Radium  C  emits  a,  /3,  and  7  rays,  and  is  half  transformed  in 
21  minutes. 

Several  hours  after  removal  from  the  emanation,  the  activity, 
whether  for  a  long  or  short  exposure,  and  whether  measured  by 
the  a,  /3,  or  7  rays,  falls  off  exponentially,  with  a  period  of  28 
minutes.  This  is  due  to  the  fact  that  the  longer  period  of  the 
rayless  product,  B,  governs  the  final  rate  of  decay,  although 
the  activity  is  really  supplied  by  the  product  C  of  21  minute 
period. 

ARE  RADIUM  A  AND  B  SUCCESSIVE  PRODUCTS? 


For  simplicity,  in  the  above  comparison  of  theory  with  ex- 
periment, the  effect  of  radium  A  on  the  subsequent  changes 
has  been  neglected.  If  radium  B  is  derived  from  radium  A, 
the  amount  of  A  present,  when  in  radioactive  equilibrium  with 
the  emanation,  is  3/28  or  .11  of  radium  B.  If  A  changes  int 


•' 

'• 


TRANSFORMATION   OF  RADIUM  115 

B  according  to  a  3  minute  period,  the  greater  part  of  A  will  be 
transformed  in  about  15  minutes,  and  it  can  be  deduced  that 
the  amount  of  B  present  after  that  interval  should  be  about 
8  per  cent  greater  than  if  A  did  not  change  into  B.  The  effect 
of  this  on  the  subsequent  decay  curves  should  be  easily  measur- 
able under  suitable  conditions.  This  point  has  been  examined 
by  the  writer,1  but  it  was  found  that  theory  and  experiment 
agreed  much  more  accurately  if  A  and  B  were  considered  to 
be  independent  products,  separately  produced  during  the  trans- 
formation of  the  emanation.  The  examination  of  the  a  ray 
curve  for  a  short  exposure  to  the  emanation  does  not  give 
definite  evidence  in  either  direction. 

The  conclusion  that  A  and  B  are  independent  products, 
however,  involves  such  important  theoretical  consequences  that 
before  accepting  it,  a  close  examination  must  be  made  to  see  if 
the  theoretical  conditions  are  completely  realized  in  practice. 

The  theory  assumes  that  radium  A  should  be  deposited  on 
the  electrode  very  shortly  after  its  production,  and  that  neither 
A  nor  its  subsequent  products  escape  from  the  electrode ;  or,  in 
other  words,  that  these  products  show  no  appreciable  volatility 
at  ordinary  temperatures. 

There  is  no  doubt,  however,  that,  under  ordinary  conditions, 
appreciable  quantities  of  both  radium  A  and  B,  and  sometimes  C, 
are  present,  mixed  with  the  emanation,  showing  that  all  of  these 
products  do  not  rapidly  diffuse  to  the  electrodes.  In  addition, 
Miss  Brooks  2  has  shown  that  radium  B  is  undoubtedly  volatile 
at  ordinary  temperatures. 

Experiments  are  at  present  in  progress  in  the  laboratory  of 
the  writer  to  decide  whether  such  divergences  between  the  theo- 
retical and  experimental  conditions  are  sufficient  to  account  for 
the  decay  curves  on  the  assumption  that  A  and  B  are  succes- 
sive products.  The  question  is  still  sub  judice,  but  it  is  hoped 
that  a  definite  answer  will  soon  be  forthcoming.3 

1  Rutherford  :  Bakerian  Lecture,  Phil.  Trans.  A,  p.  169,  1904. 

2  Miss  Brooks  :  Nature,  July  21,  1904. 

8  The  cause  of  this  discrepancy  between  theory  and  experiment  has  been  indicated 
by  some  recent  experiments  of  H.  W.  Schmidt  (Physik.  Zeit.,  6,  No.  25,  p.  897, 1905). 


I 


116  RADIOACTIVE   TRANSFORMATIONS 

If  radium  A  and  B  are  proved  to  be  independent,  it  will  be 
necessary  to  suppose  that  the  emanation  breaks  up  into  two  dis- 
tinct products,  and  also  expels  one  or  more  a  particles.  The 
observed  fact  that  the  activity  due  to  radium  A  for  a  long  ex- 
posure is  nearly  equal  to  that  of  radium  C  when  the  plates  are 
close  together,  is  in  agreement  with  both  hypotheses,  provided 
that  it  is  assumed  in  the  non-successive  hypothesis  that  each 
atom  of  the  emanation  breaks  into  two  products,  besides  expel- 
ling an  a  particle. 

On  such  a  view,  the  emanation  gives  rise  to  two  distinct 
families  of  products.  While  a  possible  change  of  this  character 
is  of  interest  and  importance,  evidence  of  an  undeniable  char- 
acter must  be  forthcoming  before  it  can  be  accepted.  If  a 
product  can  be  separated  from  radium  or  its  active  deposit, 
which  decays  exponentially,  with  a  3  minute  period,  and  does 
not  give  rise  to  radium  B  and  C,  the  independence  of  A  and  B 
will  be  completely  established. 

EFFECT  OF  TEMPERATURE  ON  THE  ACTIVE  DEPOSIT 

It  has  been  assumed  without  proof  in  the  above  discussion 
that  radium  B  rather  than  radium  C  has  the  period  of  28  minutes. 
The  comparison  of  theory  with  experiment  does  not  throw  any 
light  on  the  question,  since  the  activity  curves  are  the  same  if 
the  periods  of  the  products  B  and  C  are  interchanged. 

As  in  the  case  of  thorium,  it  is  necessary  to  have  recourse 
to  other  evidence  to  settle  whether  the  period  of  28  minutes 
belongs  to  B  or  C.  It  is  necessary  by  some  physical  or  chemi- 
cal means  to  isolate  B  from  C,  and  to  examine  separately  their 
rates  of  change. 

This  has  been  effected  by  taking  advantage  of  the  greater 

He  finds  that  radium  B  is  not  a  rayless  product,  but  emits  /3  rays  which  are  of  much 
smaller  penetrating  power  than  those  from  radium  C.  We  have  seen  (Fig.  26)  that 
the  £  ray  curve  for  a  short  exposure  to  the  emanation  passes  through  a  maximum 
35  minutes  after  removal.  This  only  holds  when  the  rays  have  been  passed  through 
a  screen  of  sufficient  thickness  to  absorb  the  £  rays  from  radium  B.  With  thinner 
screens,  the  maximum  is  reached  earlier.  When  this  new  factor  is  fully  taken  into 
account,  it  appears  probable  that  the  experimental  curves  will  completely  agree  with 
the  theory  that  radium  A,  B,  and  C  are  successive  products. 


TRANSFORMATION   OF  RADIUM  11T 

volatility  of  radium  B  when  an  active  wire  is  exposed  to  a 
high  temperature.  Miss  Gates  l  observed  that  the  active  de- 
posit of  radium  was  volatilized  at  a  white  heat,  and  redeposited 
on  the  cold  bodies  in  the  neighborhood.  Curie  and  Danne 2 
examined  this  effect  in  more  detail,  and  obtained  some  very 
interesting  results.  An  active  wire  surrounded  by  a  cold  metal 
cylinder  was  heated  for  a  short  time  by  an  electric  current,  and 
the  activity  both  of  the  wire  itself  and  of  the  interior  of  the 
cylinder  separately  examined.  At  about  400° C.,  some  of  the 
radium  B  was  volatilized.  This  was  established  by  noting 
the  variation  of  the  activity  of  the  distilled  part  after  the  heat- 
ing. This  activity  was  small  at  first,  passed  through  a  maxi- 
mum, and  then  decayed,  in  exactly  the  same  way  as  the  ft  ray 
activity  of  a  body  for  a  short  exposure  (see  Fig.  26).  This 
showed  that  the  matter  initially  deposited  on  the  cylinder  con- 
sisted only  of  the  rayless  product  B,  which  changed  into  the  ray 
product  C.  At  a  temperature  of  about  600° C.  most  of  the  B  was 
driven  off,  and  also  some  C. 

A  number  of  experiments  were  then  made  on  the  decay  of 
activity  of  the  wire  for  temperatures  varying  between  15°C. 
and  1350°C.  At  a  temperature  of  630°C.,  they  state  that  the 
activity  of  the  wire  decreased  exponentially  with  a  period  of  28 
minutes.  The  period  steadily  decreased  to  20  minutes,  while 
the  temperature  rose  to  1100° C.  At  this  temperature  it  passed 
through  a  minimum  value,  and  then  increased  to  25  minutes 
at  1300°C. 

Since  their  decay  curves  were  exponential,  Curie  and  Danne 
supposed  that  all  of  B  was  volatilized  at  630°  C.  If  this  were 
the  case,  the  results  indicated  that  the  28  minute  period  must 
be  ascribed  to  radium  C  and  the  21  minute  period  to^radium  B. 
The  experiments  also  indicated  that  rise  of  temperature  above 
630° C.  to  1100°C.  produced  an  apparent  alteration  in  the  rate 
of  change  of  radium  C.  This,  if  correct,  was  a  most  important 
result,  for  there  was  no  previous  evidence  that  temperature  had 
any  effect  in  altering  the  rate  of  transformation  of  a  radioactive 

1  Miss  Gates  :  Phys.  Rev.,  p.  300,  1903. 

2  Curie  and  Danne  :  Comptes  rendus,  cxxxviii,  p.  748,  1904. 


118  RADIOACTIVE   TRANSFORMATIONS 

product.  According  to  their  experiments,  the  rate  of  transfor- 
mation of  radium  C  was  altered  in  an  unexpected  manner  by  rise 
of  temperature,  for  it  increased  up  to  1100°C.  and  at  a  still 
higher  temperature  fell  again  nearly  to  its  normal  value. 

A  close  examination  of  the  effect  of  temperature  on  the 
active  deposit  was  recently  made  by  Dr.  Bronson 1  in  the 
laboratory  of  the  writer.  The  results  obtained  by  him  showed 
conclusively  that  a  rise  of  temperature  to  1100°  C.  has  no  effect 
in  altering  the  rate  of  transformation  of  the  active  deposit,  and 
that  the  experimental  results  obtained  by  Curie  and  Danne  can 
be  explained  by  supposing  that  in  most  of  their  experiments  the 
deposit  on  the  wire  after  heating  consisted  not  entirely  of  C  but 
of  a  mixture  of  B  and  C. 

In  order  to  test  definitely  whether  temperature  has  any  effect 
on  the  decay  of  the  active  deposit,  an  active  copper  wire  was 
placed  in  a  small  length  of  combustion  tubing,  and  the  glass 
sealed  under  diminished  pressure.  This  was  then  heated  in  an 
electric  furnace  to  different  temperatures.  The  glass  was  found 
to  withstand  a  temperature  of  about  1100°C.  The  /3  ray  activ- 
ity was  then  carefully  examined  over  a  long  interval.  Between 
2.5  and  4  hours,  the  curves  obtained  were  approximately  ex- 
ponential with  a  period  of  28  minutes.  After  6  hours,  the  curve 
was  accurately  exponential  with  a  period  of  about  26  minutes. 
Within  the  limit  of  experimental  error,  the  curves  of  decay  up 
to  1100°  C  were  found  to  be  identical  with  the  normal  decay 
curves  at  atmospheric  temperature. 

In  this  experiment,  none  of  the  distilled  products  were  able 
to  escape,  so  that  we  may  conclude  with  certainty  that  a  rise  of 
temperature  up  to  1100°C.  has  no  appreciable  effect  on  the  rate 
of  transformation  of  the  active  products. 

On  repeating  the  experiments  of  Curie  and  Danne,  Bronson 
found  that  the  decay  of  activity  after  heating  the  wire  to  a  con- 
stant temperature  was  very  variable,  and  that  approximately 
exponential  curves  were  obtained  with  periods  lying  between 
25  and  19  minutes  after  heating  the  wire  to  the  same  tempera- 
ture. If,  for  example,  a  current  of  air  was  blown  through  the 

1  Bronson  :  Amer.  Journ.  Sci.,  July,  1905. 


TRANSFORMATION   OF   RADIUM  119 

electric  furnace  before  removing  the  wire,  the  activity  fell  ex- 
ponentially with  a  period  of  about  19  minutes.  In  a  similar  way 
if  a  cold  copper  wire  was  introduced  above  the  active  wire  the 
period  had  about  the  same  value.  In  such  cases,  there  is  a  better 
opportunity  for  the  distilled  part,  radium  B,  to  escape  from  the 
wire.  Several  curves  were  obtained  which  were  accurately  ex- 
ponential with  a  period  of  19  minutes.  This  result  showed 
that  the  active  product,  radium  C,  has  a  period  of  19  minutes, 
and  that  the  26  minute  period  must  be  ascribed  to  radium  B. 

It  was  observed  that  in  all  cases  in  which  the  activity  de- 
cayed initially  with  a  period  lying  between  19  and  26  minutes, 
the  curves  were  not  at  first  accurately  exponential.  The  period 
always  tended  towards  a  value  of  26  minutes  as  the  activity  de- 
cayed, and  the  law  was  then  exponential.  This  is  exactly  what 
is  to  be  expected  if  the  activity  after  heating  the  wire  results 
from  a  mixture  of  B  and  C,  the  amount  of  C  initially  predomi- 
nating. The  activity  will  first  decay  to  half  value  with  a  period 
intermediate  between  those  of  B  and  C.  The  amount  of  radium 
B,  which  changes  more  slowly  than  C,  after  a  time  begins  to  pre- 
dominate, and  ultimately  governs  the  final  rate  of  decay,  i.  e., 
the  activity  falls  finally  according  to  an  exponential  law  with  a 
period  of  26  minutes. 

The  experiments  have  thus  shown  that  while  B  is  more  vola- 
tile than  C,  in  many  cases  all  of  the  B  is  not  removed  even  if 
the  wire  is  heated  to  a  temperature  far  above  its  point  of 
volatilization. 

The  periods  of  the  two  products,  radium  B  and  C,  are  26  and 
19  minutes  respectively,  values  somewhat  lower  than  the  periods 
28  and  21  minutes  assumed  in  the  previous  calculations.  Be- 
tween 2  and  4  hours  after  removal  from  the  emanation,  the 
activity  under  normal  conditions  falls  approximately  exponen- 
tially with  a  period  of  28  minutes,  and  this  originally  led  to  the 
choice  of  28  minutes  as  the  value  of  one  of  the  periods.  The 
decay  curve,  however,  is  not  accurately  exponential  until  about 
6  hours  after  removal  from  the  emanation,  and  the  period  is  then 
26  minutes. 

In  the  analysis  of  the  changes,  radium  C  has  been  given  the 


120  RADIOACTIVE   TRANSFORMATIONS 

shorter  period,  but  the  original  determinations  of  the  periods 
of  B  and  C  viz.,  28  and  21  minutes,  have  been  retained.  Over 
the  range  considered,  the  theoretical  curves  for  periods  of  B 
and  C  of  26  and  19  minutes  respectively  do  not  differ  much 
from  those  with  the  periods  of  28  and  21  minutes. 

The  retention  of  the  old  values  brings  out  more  clearly  the 
methods  originally  employed  of  proving  that  B  was  a  rayless 
product  and  that  C  gives  out  a,  /3,  and  7  rays.  A  more  accu- 
rate determination  of  the  various  curves  of  decay  during  the 
first  two  hours  is  at  present  in  progress. 

The  series  of  transformation  products  of  radium  which  have 
so  far  been  discussed  are  diagrammatically  shown  in  Fig.  32. 


ACTIVE  oefos/i 
FIG.  32. 
Radium  and  its  family  of  rapidly  changing  products. 

The  periods  of  the  products  are  given  for  convenience,  as  well 
as  the  character  of  the  emitted  rays. 

It  is  a  matter  of  remark  that  of  these  five  radioactive  sub- 
stances, only  radium  C  gives  out  ft  and  7  rays.  The  others 
emit  only  a  rays.  The  emission  of  the  a  rays  is,  however, 
accompanied  by  a  secondary  radiation,  which  is  produced  prob- 
ably by  the  impact  of  the  a  rays  on  matter  and  consists  of 
electrons  which  are  projected  at  a  speed  small  compared  witl\ 
that  of  the  ft  rays  proper,  and  which  are  consequently  very 
easily  deflected  by  a  magnetic  field.  The  presence  of  such  slow 
moving  electrons  was  first  observed  by  J.  J.  Thomson l  for 
radiotellurium  and  by  Rutherford2  for  radium. 

1  J.  J.  Thomson :  Proc.  Camb.  Phil.  Soc.,  Nov.  14,  1904. 

2  Rutherford:  Phil.  Mag.,  Aug.,  1905. 


TRANSFORMATION   OF  RADIUM  121 

Miss  Slater l  has  recently  shown  that  the  emission  of  a  par- 
ticles from  the  thorium  and  radium  emanations  is  also  accom- 
panied by  slow  moving  electrons  carrying  a  negative  charge. 

The  expulsion  of  such  electrons  is  probably  not  a  true  radia- 
tion from  the  active  matter  itself,  but  is  largely  a  secondary 
effect  produced  when  the  a  particles  impinge  ,upon  or  escape 
from  matter.  It  is  for  this  reason  not  advisable  to  call  them 
ft  rays,  for  this  name  should  be  retained  for  the  primary  ft  par- 
ticles emitted  from  radioactive  substances  with  velocities  ap- 
proaching that  of  light.  J.  J.  Thomson  has  suggested  that  the 
name  8  rays  be  applied  to  such  slowly  expelled  electrons. 

In  the  next  chapter,  we  shall  show  that  the  changes  in 
radium  do  not  end  with  radium  C,  but  continue  through  three 
more  distinct  stages.  The  calculations  adopted  to  analyze  the 
active  deposit  of  rapid  transformation  are  not,  however,  appre- 
ciably affected  by  the  presence  of  these  further  products,  for 
the  activity  due  to  them  is  in  most  cases  less  than  one  millionth 
of  that  observed  on  the  active  body  immediately  after  removal 
from  the  emanation. 

1  Miss  Slater:  Phil.  Mag.,  Oct.,  1905. 


CHAPTER  V 

ACTIVE  DEPOSIT   OF   RADIUM  OF  SLOW 
TRANSFORMATION 

A  BODY  which  has  been  exposed  in  the  presence  of  the 
radium  emanation  and  then  removed  does  not  completely  lose 
its  activity.  A  small  residual  activity  is  always  observed,  the 
amount  depending  not  only  on  the  quantity  of  the  emanation 
to  which  it  has  been  exposed,  but  also  upon  the  time  of  expo- 
sure. This  small  residual  activity  was  first  observed  by  Mme. 
Curie  and  has  been  closely  examined  by  the  writer. 

After  removal  from  the  emanation,  the  activity  of  a  body  at 
first  decays  according  to  the  laws  discussed  in  the  last  chapter. 
There  is  finally  an  exponential  rate  of  decay  with  a  period  of 
26  minutes.  Twenty-four  hours  after  removal,  the  active  de- 
posit of  rapid  transformation  has  disappeared  almost  completely, 
and  the  activity  left  behind  is  generally  less  than  one  millionth 
of  the  activity  observed  immediately  after  removal  from  the 
emanation. 

In  the  present  chapter,  the  variations  of  this  activity  with 
time  will  be  considered  and  the  changes  occurring  in  the  matter 
deduced.  The  active  deposit  of  slow  transformation  will  be 
shown  to  consist  of  three  successive  products  called  radium 
D,  E,  and  F.  The  analysis  of  this  apparently  insignificant 
residual  activity  observed  on  bodies  has  yielded  results  of  con- 
siderable importance.  It  has  disclosed  the  origin  of  the  radio- 
lead  of  Hofmann,  of  the  radiotellurium  of  Marckwald,  and 
also  of  the  polonium  of  Mme.  Curie;  for  these  substances  will 
be  shown  to  be  derived  from  the  transformation  of  the  radium 
atom. 

It  might  at  first  sight  be  thought  that  this  slight  residual 
activity  observed  in  bodies  was  due,  not  to  the  deposit  of  an 


SLOW   TRANSFORMATIONS   OF  RADIUM      123 

active  substance  upon  them,  but  to  a  possible  effect  produced 
by  the  powerful  radiations  of  the  emanation  to  which  the  bodies 
had  been  exposed. 

This  point  was  examined  by  the  writer1  in  the  following 
way: 

The  interior  surface  of  a  glass  tube  was  covered  with  equal 
areas  of  thin  metal,  including  platinum,  aluminium,  iron,  copper, 
silver,  and  lead.  A  large  quantity  of  emanation  was  intro- 
duced into  the  tube  and  left  there  for  seven  days.  The  activi- 
ties of  the  plates,  two  days  after  removal  from  the  emanation, 
were  separately  tested,  and  found  to  be  unequal,  being  greatest 
for  copper  and  silver  and  least  for  aluminium. 

After  standing  for  another  week,  the  initial  variations  of 
activity  had  largely  disappeared.  These  initial  differences  of 
activity  were  due  to  slight  differences  in  the  rates  of  absorption 
of  the  radium  emanation  by  the  metals.  As  this  emanation  was 
released,  the  activities  of  the  plates  reached  equal  values.  The 
radiations  from  each  plate  consisted  of  a  and  @  rays  and  were 
identical  in  penetrating  power.  This  result  shows  that  the 
residual  activity  observed  cannot  be  due  to  any  direct  actions 
of  the  radiations  on  the  body,  for  if  this  were  the  case,  we 
should  expect  the  activity  of  the  different  metals  to  vary  not 
only  in  quantity  but  in  quality.  We  may  conclude  then,  that 
the  activity  is  due  to  an  active  substance  deposited  on  the 
surface  of  the  metals.  This  view  is  completely  borne  out  by 
later  experiments,  for  it  will  be  shown  that  the  active  deposit 
can  be  removed  from  a  platinum  plate  by  solution  in  acids  and 
can  also  be  volatilized  at  a  high  temperature. 

VAKIATION  OF  THE  a  RAY  ACTIVITY  WITH  TIME 

The  a  ray  activity  of  the  body  after  reaching  a  minimum 
value  during  the  first  few  days,  steadily  increases  in  amount 
for  several  years.  The  activity  during  the  first  few  months 
increases  nearly  proportionally  with  the  time.  The  curve 
(Fig.  33)  then  begins  to  bend  over,  and  after  240  days  —  the 
period  over  which  it  has  so  far  been  examined  —  becomes  much 

i  Rutherford :  Phil.  Mag.,  Nov.,  1904. 


124 


RADIOACTIVE   TRANSFORMATIONS 


more  flattened  and  obviously  approaches  a  maximum  value.  The 
explanation  of  this  rise  of  activity  will  be  considered  at  a  later 
stage. 

VARIATION  OF  THE  ft  RAY  ACTIVITY  WITH  TIME 

The  residual  activity  initially  comprises  both  a  and  ft  rays, 
the  latter  being  present  in  a  much  greater  relative  proportion 


FIG.  33. 

Rise  of  a  ray  activity  of  a  body  after  exposure  to  the  radium  emanation, 
activity  is  a  measure  of  the  amount  of  radium  F  present. 


The 


than  is  observed  in  radium  or  uranium.  The  ft  ray  activity  is 
small  at  first,  but  increases  with  time,  reaching  a  maximum 
after  about  50  days.  The  variation  of  activity  with  time  is 
shown  in  Fig.  34.  A  plate  was  exposed  for  3.75  days  to  the 
radium  emanation,  and  the  observations  of  the  ft  ray  activity, 
by  means  of  an  electroscope,  were  begun  24  hours  after  remo- 
val. The  time  was  measured  from  the  middle  of  the  period  of 
exposure  to  the  emanation.  The  curve  is  seen  to  be  similar 


SLOW   TRANSFORMATIONS   OF   RADIUM       125 

in  shape  to  the  recovery  curve  of  the  emanation  or  of  ThX. 
The  /3  ray  activity  It  at  any  time  t  after  removal  is  given  by 


The  activity  reaches  half  value  in  about  6  days,  and  after  50 
days  has  nearly  reached  a  maximum. 


J 


,5                 /o               is               s.o              £0  -90  oo 
TIME:  IN  DAYS 

FIG.  34. 

Rise  of  $  ray  activity  of  a  body  after  exposure  to  the  radium  emanation.    The 
ft  ray  activity  is  a  measure  of  the  amount  of  radium  E  present. 

Observations  of  the  fi  ray  activity  were  continued  for  18 
months,  but  showed  that  the  activity  remained  practically  con- 
stant after  50  days. 

A  curve  of  this  character  indicates  that  the  /3  ray  product 
is  produced  at  a  constant  rate  from  a  primary  source,  whose 
rate  of  transformation  is  so  slow  as  to  appear  nearly  constant 
over  the  period  of  observation.  It  follows  from  the  curve  that 
the  {3  ray  product  has  a  period  of  6  days. 

The  fact  that  the  a  and  fj  ray  activities  increase  almost  from 


126  RADIOACTIVE   TRANSFORMATIONS 

zero  shows  that  their  primary  source,  called  radium  D,  is  a  ray- 
less  product,  which,  as  we  shall  see  later,  is  probably  half  trans- 
formed in  about  40  years.  Radium  D  is  transformed  into  the 
ft  ray  product  called  radium  E,  which  is  half  transformed  in 
about  6  days. 

EFFECT  OF  TEMPERATURE  ON  THE  ACTIVITY 

A  platinum  plate  several  months  after  its  removal  from  the 
emanation  was  placed  in  an  electric  furnace  and  heated  for  a  few 
minutes  to  varying  temperatures.  Exposure  for  four  minutes, 
first  at  430°C.  and  later  at  800°C.,  had  little  if  any  effect  in 
altering  either  the  a  or  ft  ray  activity.  The  a  ray  activity  was, 
however,  almost  completely  removed  by  heating  the  plate  to 
about  1050°  C.,  while  the  ft  ray  activity  did  not  at  the  time  show 
any  change.  This  result  shows  clearly  that  the  product  which 
emits  a  rays  is  more  volatile  than  the  product  which  emits 
ft  rays. 

This  experiment  is  another  example  of  the  way  in  which 
differences  in  volatility  of  two  products  may  be  utilized  to 
effect  a  partial  separation  of  one  from  the  other. 

We  now  come  to  another  striking  observation.  The  ft  ray 
activity  of  the  platinum  plate,  though  apparently  unchanged 
immediately  after  the  heating,  began  slowly  to  decrease,  and 
finally  fell  to  one  quarter  of  the  initial  value.  Subtracting  this 
residual  activity,  it  was  found  that  the  ft  ray  product  lost  its 
activity  exponentially,  falling  to  half  value  in  about  4.5  days. 

We  may  thus  conclude  that  the  heating  of  the  active  deposit 
had  a  double  action ;  for  not  only  was  the  a  ray  product  (which 
will  be  shown  to  arise  from  the  ft  ray  product,  radium  E)  driven 
off,  but  about  three  quarters  of  the  primary  source,  radium  D, 
was  also  volatilized. 

We  thus  have  the  striking  result  that  in  a  mixture  of  three 
successive  products,  the  first  and  third  are  mostly  volatilized 
at  a  temperature  of  about  1000° C.,  while  the  middle  product  is 
unaffected.  It  will  be  observed  that  the  period  of  decay  of  the 
ft  ray  product  (4.5  days),  observed  after  heating,  does  not  agree 
with  the  period  of  the  same  product  (6  days)  deduced  from  the 


SLOW   TRANSFORMATIONS   OF   RADIUM      12T 

rise  curve  of  Fig.  33.  This  difference  requires  further  inves- 
tigation. The  6  day  period  is  probably  the  more  correct  value 
under  normal  conditions. 

SEPARATION  OF  THE  a  RAY  PRODUCT  BY  BISMUTH 

The  emanation  from  30  milligrams  of  radium  bromide  was 
condensed  in  a  glass  tube  and  left  there  for  one  month.  The 
active  deposit  remaining  on  the  surface  of  the  glass  was  then 
dissolved  in  dilute  sulphuric  acid  and  the  solution  laid  by  for 
about  a  year.  During  this  interval  the  a  ray  activity  steadily 
increased.  By  introducing  a  polished  bismuth  disc  into  the 
solution,  the  a  ray  product  may  be  deposited  electrochemically 
on  the  bismuth.  By  introducing  several  bismuth  discs  suc- 
cessively into  the  solution  and  allowing  them  to  remain  for 
several  hours,  the  a  ray  product  was  mostly  removed.  On 
evaporating  the  solution  to  dry  ness,  it  was  found  that  only  10 
per  cent  of  the  original  a  ray  activity  remained  behind. 

The  @  ray  activity  of  the  solution  was  not  altered  by  this 
process.  The  bismuth  discs  gave  out  only  a  rays,  but  no  trace 
of  /3  rays.  This  result  shows  that  only  the  a  ray  product  was 
removed.  Radium  D,  as  well  as  E,  was  left  behind,  for  if  some 
radium  D  had  been  deposited  on  the  bismuth,  it  would  have 
changed  into  radium  E,  and  consequently  some  ft  rays  would 
have  been  emitted  from  the  bismuth  disc,  after  standing  for 
several  weeks  in  order  to  allow  time  for  D  to  change  into  E. 

No  such  effect,  however,  was  observed.  The  activities  of 
these  bismuth  discs  were  tested  in  an  a  ray  electroscope  for 
over  200  days.  The  activity  of  each  was  found  to  fall  off 
nearly  exponentially,  reaching  half  of  the  initial  value  after 
about  143  days.  We  may  thus  conclude  that  the  substance 
which  emits  a  rays  is  a  simple  product,  which  is  half  trans- 
formed in  143  days.  This  a  ray  product  will  be  called  radium 
F,  for  it  will  be  shown  to  be  a  successive  product  of  radium  E. 

The  fact  that  radium  E  is  the  parent  of  F  is  clearly  brought 
out  by  the  following  experiment. 

A  platinum  wire  coated  with  the  active  deposit  of  slow 
transformation  was  exposed  for  some  minutes  to  a  temperature 


128 


RADIOACTIVE   TRANSFORMATIONS 


of  over  1000° C.  Most  of  the  radium  F  was  volatilized.  The  a 
ray  activity  of  this  platinum  plate  was  then  carefully  examined 
for  several  weeks.  The  small  a  ray  activity,  observed  imme- 
diately after  removal  from  the  furnace,  increased  rapidly  during 
the  first  two  weeks  and  then  more  slowly.  The  gain  of  a  ray 
activity  with  time  is  shown  in  Fig.  35. 


/COj 


60 


80 


-    TIMS  IN  OAYQ  - 

FIG.  35. 

Hise  of  a  ray  activity  on  a  platinum  plate  which  has  been  heated  to  a  tempera- 
ture sufficient  to  remove  most  of  the  radium  D  and  F  present.  Radium  E  is 
left  behind  and  changes  into  F. 

A  curve  of  this  character  is  to  be  expected  if  radium  E  is  the 
parent  of  F.  The  action  of  the  high  temperature  volatilized 
the  greater  part  of  D  and  F,  but  left  E  behind.  The  radium 
E  was  then  transformed  with  a  period  of  4.5  days  and  changed 
into  F.  The  a  ray  activity  thus  initially  rose  rapidly,  due  to 
the  new  radium  F  supplied  by  the  transformation  of  E.  The 
slow  increase  observed  after  some  weeks,  when  most  of  the 
radium  E  present  had  been  transformed,  was  due  to  the  pro- 
duction of  radium  F  by  the  small  amount  of  D  and  E  which 
was  not  volatilized  from  the  platinum  plate. 

We  may  thus  conclude  that  radium  E  is  the  parent  of 
radium  F. 


SLOW   TRANSFORMATIONS   OF   RADIUM       129 


It  has  been  previously  shown  that  radium  E  is  produced  from 
radium  D,  which  itself  does  not  emit  /3  rays.  The  small  a  ray 
activity  observed  initially  when  radium  D  is  present  in  maxi- 
mum amount  shows  that  radium  D  does  not  emit  a  rays. 
Radium  D  is  consequently  a  rayless  product. 

SUMMARY  OF  THE  RADIUM  PRODUCTS 

The  analysis  of  the  active  deposit  of  slow  change  has  thus 
disclosed  the  existence  of  three  successive  products  of  radium. 
The  period  of  change  of  these  products  and  some  of  their  dis- 
tinctive physical  and  chemical  properties  are  tabulated  below. 


Product. 

Time  to  be  half 
transformed. 

Rays. 

Chemical  and  physical  properties. 

Radium  D 

About  40  years 

None 

Soluble  in  strong  acids,  volatilized 

at  or  below  1000°  C. 

"       E 

6  days 

$  and  (7?) 

Non-volatile  at  1000°  C. 

«       F 

143    « 

a 

Volatile  at  about  1000°  C.,  depos- 

ited from  a  solution  on  a  bis- 

muth plate. 

The  method  for  deducing  the  period  of  radium  D  will  be  con- 
sidered a  little  later.  A  sufficient  amount  of  radium  E  has 
not  yet  been  collected  to  test  whether  it  gives  out  7  as  well  as 
/3  rays.  But  since  in  every  other  substance  examined,  these 
two  types  of  rays  always  go  together,  it  is  almost  certain  that 
radium  E  gives  out  7  rays. 

In  the  last  chapter  it  was  shown  that  the  active  deposit  of 
rapid  change  consists  of  the  three  successive  products,  radium 
A,  B,  and  C.  It  is  thus  natural  to  conclude  that  radium  D  is 
derived  directly  from  the  transformation  of  radium  C.  It  is 
difficult  to  show  definitely  that  radium  C  is  the  parent  of  D. 
We  know,  however,  that  D  must  be  derived  from  either  the 
emanation  or  one  of  its  products,  and  since  the  products  A,  B, 
and  C  are  lineal  descendants  of  the  emanation,  the  most  plausi- 
ble assumption  is  that  the  family  of  products  D,  E,  and  F  are 
also  lineal  descendants  of  radium  C. 

On  this  assumption,  the  various  radium  products,  together 

9 


130  RADIOACTIVE   TRANSFORMATIONS 

with   their  periods   of  transformation   and   the   types   of  rays 
emitted,  are  shown  diagrammatically  below  (Fig.  36). 

It  is  instructive  for  a  moment  to  review  briefly  the  series  of 
changes  exhibited  by  radium.  The  radium  atom  is  a  compara- 
tively stable  one,  and  on  an  average  only  half  the  radium  atoms 
break  up  in  1300  years.  The  a  particle  projected  during  the 
disintegration  of  the  atom  has  a  velocity  slower  than  those  from 
the  radium  products,  and  can  only  pass  through  3.  5  cms.  of  air 
before  complete  absorption.  The  radium  suffers  a  radical  change 
on  account  of  the  loss  of  the  a  particle,  and  is  transformed  into 
a  gas  —  the  radium  emanation  —  which  is  far  more  unstable 
than  radium  itself,  for  half  of  it  breaks  up  in  3.8  days.  After 
the  expulsion  of  an  a  particle,  the  product  radium  A  makes 


«  >  »  "*  <i  ^  v 

<5-d-6-o-  o-  o-  c£  6- 


EMAN.       /?A»A        f^AaB        RAD.C       RADD 

iJhJirjS. 


A *»/o CHANGE  ACTIV 

FIG.  36. 
Radium  and  its  family  of  products. 

its  appearance.     This  is  the  most  unstable  of  all  the  radium 
products,  for  half  of  it  breaks  up  in  3  minutes. 

The  next  product  is  radium  B,  with  a  period  of  26  minutes. 
It  has  the  peculiarity  of  being  transformed  without  the  emission 
of  rays  at  all.  This  points  either  to  a  transformation  by  the 
rearrangement  of  the  components  of  the  atom  without  any  loss 
of  mass,  or,  as  is  more  likely,  to  the  emission  of  an  a  particle  at 
a  velocity  too  low  to  ionize  a  gas.  It  will  be  seen  later  that  the 
a  particle  loses  the  property  of  ionization  when  its  velocity  falls 
below  about  one  fortieth  of  the  velocity  of  light,  so  that  the  a 
particle  may  be  expelled  at  a  considerable  speed  and  yet  show 
no  ionization  effects.  The  next  substance  is  radium  C,  which 
is  the  most  remarkable  of  all  the  radium  products,  for  in 
breaking  up  it  emits  all  three  types  of  rays.  It  would  appear 


SLOW   TRANSFORMATIONS   OF  RADIUM      131 

as  if  the  transformation  of  C  were  accompanied  by  a  most 
violent  explosion  in  the  atom,  for  not  only  is  the  a  particle 
ejected  with  a  greater  speed  than  from  any  of  the  other  prod- 
ucts, but  at  the  same  time  @  particles  are  expelled  with  a 
velocity  nearly  equal  to  that  of  light.  There  is  also  an  emission 
of  very  penetrating  7  rays. 

The  a  particle  projected  from  radium  C  can  traverse  7  cms. 
of  air  before  complete  absorption,  while  the  a  rays  from  the 
other  products  have  a  range  not  greater  than  4.8  cms.  After 
this  violent  atomic  outburst,  the  residual  atom,  radium  D,  is  far 
more  stable  and  breaks  up  without  the  appearance  of  rays. 

The  next  product  is  radium  E,  which  emits  only  @  and  y 
rays.  It  has  a  comparatively  short  life,  but  gives  rise  in  turn  to 
radium  F,  which  has  a  slow  period  of  change.  No  further  prod- 
ucts of  transformation  have  been  detected,  and  the  interesting 
question  of  the  final  or  end  product  of  radium  is  reserved  for 
discussion  in  Chapter  VIII. 

PERIOD  OF  CHANGE  OF  RADIUM  D 

Radium  D  does  not  emit  rays,  and  consequently  neither  its 
properties  nor  its  rate  of  transformation  can  be  determined  by 
direct  means.  The  following  product,  radium  E,  however, 
emits  y3  rays,  and  by  noting  the  variations  of  its  activity  when 
in  equilibrium  we  should  be  able  to  detect  any  variation  in  the 
rate  of  change  of  the  parent  product  D. 

This  is  readily  seen  to  be  the  case,  for  when  equilibrium  is 
reached,  the  number  of  atoms  of  E  which  break  up  per  second 
will  always  be  equal  to  the  number  of  atoms  of  D  breaking  up 
per  second.  Unfortunately  the  rate  of  transformation  of  D  is 
so  slow  that  no  certain  change  in  the  equilibrium  activity  of  E 
has  been  detected  in  the  course  of  one  year,  and  a  long  interval 
of  time  will  probably  be  necessary  to  fix  the  period  of  D  by 
direct  measurement. 

It  is  of  importance  to  form  a  rough  estimate  of  its  probable 
period.  This  can  be  deduced  on  certain  assumptions  which 
are  probably  approximately  realized  in  practice. 

Suppose  that  a  quantity  of  emanation  is  introduced  into  a 


132  RADIOACTIVE   TRANSFORMATIONS 

closed  vessel.  and  left  there  to  decay.  Several  hours  after  the 
introduction  of  the  emanation,  the  amount  of  radium  C  which 
emits  ft  rays  reaches  a  maximum  value,  and  then  decays  at  the 
same  rate  as  the  emanation.  If  ql  is  the  maximum  number  of 
ft  particles  emitted  per  second  from  radium  C  at  its  maximum 
activity,  the  total  number  -ZVj  emitted  during  the  life  of  the  ema- 

nation is  very  approximately  given  by  -ZVi  =  —  ,  where  Xx  is  the 

A! 

constant  of  change  of  the  emanation.  Suppose  that  the  active 
deposit  of  slow  transformation  is  allowed  to  remain  undisturbed 
for  about  50  days  after  the  emanation  has  practically  disappeared. 
Radium  D  and  E  will  then  be  in  equilibrium.  Let  q2  be  the 
number  of  ft  particles  emitted  from  D  and  E.  Then  if  the 
transformation  of  radium  D  follows  the  ordinary  exponential 
law  with  a  constant  X2,  the  total  number  N2  of  ft  particles 
emitted  during  the  life  of  radium  D  is  given  as  before  by 

N2  =  —.     But    if    each   atom   of    radium    C   in   breaking   up 
A2 

emits  one  ft  particle,  the  total  number  of  ft  particles  emitted 
during  the  life  of  the  emanation  must  be  equal  to  the  number 
of  atoms  of  emanation  originally  present.  The  number  of  atoms 
of  D  formed  by  the  emanation  will  also  be  equal  to  this  quan- 
tity, and  if  each  atom  of  D  gives  rise  to  one  of  E,  which  breaks 
up  with  the  expulsion  of  one  ft  particle,  we  see  that  the  total 
number  of  ft  particles  expelled  from  C  during  the  life  of  the 
emanation  must  be  equal  to  the  total  number  of  particles 
expelled  from  E  during  the  life  of  D.  Consequently  JV^  =  -ZV2> 
and  therefore 


It  is  not  easy  to  measure  directly  the  number  of  ft  particles 
expelled  either  from  radium  C  or  E,  but  on  the  assumption  that 
the  average  ft  particle  emitted  from  C  or  E  produces  about  the 
same  ionization  in  a  gas, 


SLOW   TRANSFORMATIONS   OF  RADIUM      133 

where  ?\,  iz  are  the  saturation  ionization  currents  due  to  C  and  E 
respectively,  measured  under  the  same  conditions  in  the  same 

testing  vessel.     This  ratio,  -^,  can  be  readily  determined,  so  that 

A  ^1 

the  ratio  -2  is  known.      Substituting   the  value  of  Xx  for  the 

emanation,  X2  can  be  determined. 

Proceeding  by  this  method,  the  writer  1  deduced  that  radium 
D  should  be  half  transformed  in  about  40  years.  This  period 
is  almost  certainly  of  the  right  order,  but  from  the  nature  of  the 
assumptions,  the  value  cannot  pretend  to  be  more  than  a  first 
approximation  to  the  truth.  The  main  source  of  error  probably 
lies  in  the  assumption  that  the  /3  particles  of  radium  C  and  E 
produce  the  same  average  ionization  in  the  gas. 

As  a  criterion  of  the  order  of  accuracy  obtained  in  predicting 
periods  of  change  by  these  means,  it  may  be  mentioned  that,  by 
a  similar  method,  I  deduced  that  the  period  of  radium  F  was 
about  one  year.  Actual  observation  has  since  shown  that  this 
period  is  143  days.  I  think  that  the  period  of  D  will  certainly 
be  found  to  lie  between  20  and  80  years. 

VARIATION  OF  THE  a  AND  0  RAY  ACTIVITY  OVER  LONG 
PERIODS  OF  TIME 

We  are  now  in  a  position  to  deduce  the  variation  of  the  a 
and  ft  ray  activity  for  the  active  deposit  over  long  intervals  of 
time.  Since  radium  E  is  transformed  at  a  rapid  rate  compared 
with  F,  we  may  assume  as  a  first  approximation  that  D  is  trans- 
formed directly  into  F.  The  problem  thus  reduces  to  the  fol- 
lowing: Given  that  the  periods  of  two  successive  products  are 
40  years  and  143  days  respectively,  find  the  number  of  atoms 
of  each  product  present  at  any  time.  This  is  exactly  equivalent 
to  the  practical  case  already  considered  on  page  50  for  the  active 
deposit  of  thorium  where  the  two  changes  had  periods  of 
11  hours  and  55  minutes  respectively. 

The  ft  ray  activity  of  D  and  E  after  reaching  its  maximum 
will  decrease  exponentially,  falling  to  half  value  in  40  years. 

1  Kutherford,  Phil.  Mag.,  Nov.,  1904. 


134 


RADIOACTIVE   TRANSFORMATIONS 


Using  the  equation  discussed  on  page  51,  it  can  at  once  be 
deduced  that  the  number  of  atoms  of  radium  F  reaches  its  maxi- 
mum in  about  2.6  years  and  that  this  substance  will  ultimately 
decay  pari  passu  with  the  parent  product  D,  i.  e.,  it  will  be 
half  transformed  about  40  years  later.  The  curves  shown  in 
Fig.  37  give  the  relative  number  of  atoms  of  E  and  F  which 
break  up  per  second  at  any  time  after  the  formation  of  the 


100                ^_ 

fiftj 

E 

——-  —  —  —  . 

^s-* 

— 

=r= 

=== 

=== 

=== 

60J         / 

J/ 

Jf 

L 

-  lYtfitf  « 

}r<rara  — 

1                 2345678 

FIG.  37. 

Curve  E  E  represents  the  variation  in  the  number  of  atoms  of  radium  D  break- 
ing up  per  second.  Curve  F  F  represents  the  number  of  atoms  of  radium  F 
breaking  up  per  second. 

deposit.  Since  the  activity  of  F  is  proportional  to  the  number 
of  atoms  of  F  which  break  up  per  second,  we  see  that  the  activ- 
ity of  F  will  rise  from  zero  to  a  maximum  after  2.6  years,  and 
will  then  decay  with  a  40  year  period. 

The  variation  of  the  a  ray  activity  with  time,  is  in  good 
agreement  with  the  theoretical  curve  over  the  range  so  far 
examined  (See  Fig.  33). 

It  is  of  interest  to  note  that  the  same  a  ray  activity  observed 


SLOW   TRANSFORMATIONS   OF   RADIUM      135 

9  days  after  the  formation  of  the  active  deposit  is  again  reached 
after  an  interval  of  about  180  years. 

The  production  by  radium  of  the  active  deposit  of  slow  trans- 
formation at  once  explains  the  strong  radioactivity  observed  in 
rooms  in  which  large  quantities  of  radium  have  been  used,  even 
when  the  radium  has  been  completely  removed  for  some  time. 
This  effect  has  been  observed  by  several  experimenters,  and 
especially  by  those  who  have  been  occupied  in  the  separation 
and  concentration  of  large  quantities  of  radium. 

The  radium  emanation  released  from  the  radium  is  trans- 
ferred by  diffusion  and  convection  currents  throughout  the 
whole  laboratory,  and  far  distant  rooms  into  which  radium  prep- 
arations have  not  been  introduced  become  permanently  radio- 
active. The  emanation  is  transformed  in  situ  through  the 
succession  of  products  radium  A,  B,  and  C,  and  finally  passes 
into  the  active  deposits  of  slow  change.  This  matter  is  de- 
posited on  the  interior  surface  of  rooms  and  on  every  object  in 
the  building.  For  a  given  supply  of  emanation,  the  a  ray 
activity  will  be  small  at  first,  but  will  steadily  increase  for  about 
three  years. 

This  residual  activity  on  bodies  is  a  source  of  considerable 
disturbance  in  radioactive  work.  Eve,1  for  example,  found 
that  every  substance  examined  in  the  Macdonald  Physical 
Laboratory  of  McGill  University  showed  an  abnormally  large 
natural  activity.  At  the  time  of  testing,  this  activity  was 
about  60  times  greater  than  that  observed  in  the  same  labora- 
tory before  the  introduction  of  large  quantities  of  radium  into 
the  building.  All  electroscopes  made  of  materials  exposed  in 
the  building  had  a  large  natural  leak  due  to  the  active  deposit. 
This  can  in  part  be  removed  by  cleaning  with  sandpaper  or  by 
solution  in  acids.  Unless  all  electroscopes  or  testing  vessels 
are  made  outside  the  laboratory  to  insure  a  small  natural  leak, 
measurement  of  very  weak  radioactivities  is  rendered  almost 
impossible.  When  once  a  building  has  been  infected,  it  does 
not  serve  any  immediate  purpose  to  remove  the  radium,  for  the 

i  Eve:  Nature,  March  16,  1905. 


136  RADIOACTIVE   TRANSFORMATIONS 

a  ray  activity  will  continue  to  increase  for  about  three  years 
and  will  last  for  hundreds  of  years. 

For  these  reasons  it  is  very  advisable  to  reduce  the  escape  of 
emanation  into  the  air  of  a  laboratory  as  far  as  possible  and  to 
keep  all  radium  salts  in  sealed  vessels. 

PRESENCE  OF  THE  ACTIVE  DEPOSIT  IN  RADIUM 

Since  radium  D  is  produced  from  radium  at  a  nearly  constant 
rate,  it  should  gradually  increase  in  quantity  with  the  age  of 
the  radium.  The  presence  of  radium  D  in  old  radium  can  be 
detected  in  a  very  simple  way.  With  a  freshly  prepared  sample 
of  radium,  continued  boiling  for  five  or  six  hours  removes  the 
emanation  as  fast  as  it  is  formed,  and  reduces  the  /3  ray  activity 
arising  from  radium  C  to  a  fraction  of  one  per  cent  of  its 
maximum  value  when  in  radioactive  equilibrium. 

A  very  different  result  is  observed  if  an  old  preparation  of 
radium  is  treated  in  a  similar  way.  The  writer  had  in  his 
possession  a  small  quantity  of  impure  radium  kindly  presented 
by  Professors  Elster  and  Geitel  four  years  before. 

After  continued  boiling,  the  activity  could  not  be  reduced 
below  8  per  cent  of  the  original  amount,  or  about  9  per  cent  of 
the  activity  due  to  C  alone.  This  residual  /3  ray  activity  was 
due  to  the  radium  E  stored  up  in  the  compound. 

The  amount  of  radium  E  will  steadily  increase  with  time,  and 
will  reach  a  maximum  value  when  the  same  number  of  atoms  of 
radium  C  and  radium  E  break  up  per  second.  The  number  of 
/B  particles  expelled  per  second  from  radium  E  will,  under  such 
conditions,  be  equal  to  the  number  expelled  from  radium  C. 
Since  the  radium  itself  is  transformed  very  slowly  compared  with 
radium  D,  the  amount  of  radium  D  produced  per  year  (measured 
by  the  y5  ray  activity  of  radium  E)  should  be  about  1.7  per  cent 
of  the  equilibrium  amount. 

The  0  ray  activity  due  to  radium  E  should  thus  be  about 
7  per  cent  of  that  due  to  radium  C  after  the  lapse  of  four  years. 
The  observed  and  calculated  values  (9  and  7  per  cent  respec- 
tively) are  thus  in  fair  agreement. 

By  adding  a  trace  of  sulphuric  acid  to  the  radium  solution, 


SLOW   TRANSFORMATIONS   OF  RADIUM       137 

the  radium  was  precipitated  and  the  products  D,  E,  F,  which 
are  soluble  in  sulphuric  acid,  remained  in  the  solution.  The 
filtrates  thus  contained  a  large  part  of  the  above  three  products. 
The  radium  F  was  removed  from  the  solution  by  means  of 
bismuth  discs,  and  showed  an  activity  to  be  expected  from  the 
age  of  the  radium. 

VARIATION  OF  THE  ACTIVITY  OF  RADIUM  WITH  TIME 

We  shall  see  later  that  radium  itself,  apart  from  its  products, 
is  probably  half  transformed  in  about  1300  years,  and  conse- 
quently the  number  of  atoms  breaking  up  per  second  decreases 
exponentially  according  to  this  period.  In  consequence  of  the 
formation  of  the  active  deposit  of  slow  change,  the  activity 
supplied  by  it  will  at  first  more  than  compensate  for  the  de- 
crease in  the  activity  of  the  radium  itself.  The  activity  will 
rise  for  several  hundred  years,  but  will  ultimately  decay  expo- 
nentially with  the  period  of  the  radium. 

When  sufficient  time  has  elapsed  for  approximate  equilibrium 
between  the  mixture  of  products,  the  number  of  /3  particles 
expelled  from  the  old  radium  will  be  twice  that  due  to  radium 
C  alone;  for  radium  E  will  emit  per  second,  under  such  con- 
ditions, the  same  number  of  /3  particles  as  radium  C. 

It  can  readily  be  calculated  from  the  theory  of  two  changes, 
that  the  number  of  $  particles  expelled  from  radium  and  its 
products  will  steadily  increase  until  a  maximum  is  reached 
after  226  years.  After  that  period,  the  number  will  decrease 
nearly  exponentially  with  a  period  of  1300  years. 

The  variation  with  time  of  the  number  of  /3  particles  ex- 
pelled from  radium  is  shown  in  Fig.  38,  Curve  BB. 

Radium  and  its  family  of  rapidly  changing  products  together 
emit  four  a  particles  for  the  one  emitted  from  radium  F.  By 
calculation  it  can  be  shown  that  the  number  of  a  particles 
expelled  from  radium  will  reach  a  maximum  after  about  111 
years,  and  will  then  be  about  1.19  times  the  number  emitted 
from  radium  about  one  month  old.  The  number,  as  in  the  case 
of  the  (3  particles,  will  then  decrease  and  the  period  will  be 
1300  years. 


138 


RADIOACTIVE   TRANSFORMATIONS 


Curve  AA  shows  the  variation  with  time  of  the  number  of  a 
particles  expelled  from  radium  and  its  mixture  of  products. 
Curve  CC  represents  the  number  of  radium  atoms  breaking  up 
per  second. 

These  calculations  of  the  variation  of  the  activity  of  radium 
with  time  depend  upon  the  accuracy  of  the  periods  of  change  of 


too 


eo 


n 

L 


200 


400 


600  800 

TIME  IN  YEARS. 


/OOO 


/200 


FIG.  38. 

Curve  A  A  represents  the  variation  with  time  in  the  number  of  a  particles  ex- 
pelled from  radium.  Curve  B  B  represents  the  number  of  0  particles  expelled 
per  second.  Curve  C  C  represents  the  number  of  atoms  of  radium  breaking 
up  per  second. 

radium  and  radium  D.     Any  alteration  in  these  values  will  to 
some  extent  alter  the  curves  of  variation  of  activity  with  time. 

IDENTITY  OF  RADIUM  F  WITH  RADIOTELLURITJM 

Since  the  products  D,  E,  and  F  are  continuously  produced  by 
radium,  they  should  be  found  in  all  radioactive  minerals  con- 


SLOW   TRANSFORMATIONS   OF   RADIUM      139 

taining  radium,  and  in  amounts  proportional  to  the  amount  of 
radium  in  the  mineral.  It  is  now  necessary  to  consider  whether 
any  of  these  products  have  been  previously  separated  from 
radioactive  minerals  and  known  by  other  names. 

We  shall  first  consider  the  product  radium  F,  which  will  be 
shown  to  be  identical  with  the  very  active  substance  called 
radiotellurium,  separated  by  Marckwald  from  pitchblende  resi- 
dues. In  endeavoring  to  establish  the  indentity  of  two  prod- 
ucts, the  main  criteria  to  be  relied  upon  are :  — 

(1)  the  identity  of  the  radiations  or  characteristic  emanations 
emitted  by  the  products ; 

(2)  the  identity  of  the  periods  of  change  of  the  products ; 

(3)  the  similarity  of  chemical  and  ph}Tsical  properties  of  the 
active  products. 

The  third  criterion  is  initially  of  less  importance  than  (1)  or 
(2),  since  in  most  cases  the  active  products  are  separated  in  a 
very  impure  state  and  the  apparent  chemical  reaction  may  be 
largely  modified  by  the  presence  of  impurities. 

We  have  seen  that  the  product  radium  F  emits  only  a  rays, 
has  a  period  of  about  143  days,  and  is  deposited  on  bismuth 
from  the  active  solution.  Radiotellurium  behaves  in  identically 
the  same  manner.  In  addition,  the  writer :  directly  compared 
the  rates  of  decay  of  the  activities  of  radiotellurium  and  radium  F 
and  found  them  to  be  the  same  within  the  limit  of  experimental 
error.  Each  loses  half  its  activity  in  about  143  days.  The 
period  of  decay  of  radiotellurium  has  also  been  experimentally 
examined  by  Meyer  and  Schweidler  and  Marckwald.  The  former 
found  a  period  of  135  days  and  the  latter  139.  Considering  the 
difficulty  of  making  accurate  comparative  measurements  over 
such  long  intervals  of  time,  the  values  obtained  by  the  different 
observers  are  in  remarkably  good  agreement. 

The  writer  also  found  that  the  rays  emitted  from  radium  F 
had  the  same  penetrating  power  as  those  emitted  from  an  active 
bismuth  plate  coated  with  radiotellurium.  It  is  known  from 
the  work  of  Bragg  and  others,  that  each  product  of  radium 
emits  a  rays  of  a  penetrating  power,  which  is  definite  for  each, 

1  Rutherford  :  Phil.  Mag.,  Sept.,  1905. 


140  RADIOACTIVE   TRANSFORMATIONS 

but  varies  considerably  among  the  different  products.  This 
equality  in  penetrating  power  thus  supplies  strong  evidence  in 
favor  of  the  identity  of  the  two  products. 

We  may  thus  conclude  that  the  radiotellurium  of  Marckwald 
contains  as  its  active  constituent  the  product  radium  F;  or,  in 
other  words,  radiotellurium  is  a  transformation  product  of 
radium. 

The  methods  of  separation  and  concentration  of  radiotel- 
lurium used  by  Marckwald  are  of  special  interest.  The  sepa- 
ration by  Mme.  Curie  of  radium  from  pitchblende  in  which  it 
existed  in  the  proportion  of  less  than  one  part  in  a  million  was 
in  itself  a  notable  performance,  but  the  work  of  Marckwald  in 
the  separation  of  radiotellurium  constitutes  a  still  more  strik- 
ing illustration  of  the  possibility  of  chemically  concentrating  a 
radioactive  substance  existing  in  almost  infinitesimal  amount. 

Marckwald  initially  observed  that  a  bismuth  rod  dipped  into 
a  solution  of  pitchblende  residues  became  coated  with  a  deposit 
which  emitted  only  a  rays.  After  some  days,  the  active  sub- 
stance was  in  this  way  almost  completely  removed  from  the 
solution  and  obtained  on  the  bismuth.  The  deposit  on  the 
bismuth  was  found  to  consist  for  the  most  part  of  tellurium, 
and  for  this  reason  Marckwald  called  the  active  substance  radio- 
tellurium.  Later  Marckwald  devised  very  simple  and  efficient 
means  of  separating  the  active  matter  from  the  tellurium,  and 
finally  obtained  a  substance  which,  weight  for  weight,  was  far 
more  active  than  radium. 

Five  tons  of  uranium  residues,  corresponding  to  15  tons  of 
the  Joachimsthal  mineral,  were  worked  up  to  extract  the  radio- 
tellurium  from  it,  and  he  finally  obtained  only  3  milligrams  of 
the  active  substance.  If  plates  of  tin,  copper,  or  bismuth  were 
dipped  into  a  hydrochloric  acid  solution  of  this  substance, 
they  were  found  to  be  covered  with  a  finely  divided  deposit. 
These  plates  were  extremely  radioactive,  and  gave  marked  ioniz- 
ing, photographic,  and  phosphorescent  effects.  As  an  illustra- 
tion of  the  enormous  activity  of  this  substance,  Marckwald  states 
that  a  weight  of  1/100  of  a  milligram  deposited  on  a  copper  plate 
4  sq.  cms.  in  area  lighted  up  a  zinc  sulphide  screen  brought 


SLOW   TRANSFORMATIONS   OF  RADIUM      141 

near  it  so  strongly  that  the  luminosity  could  be  clearly  seen  by 
an  audience  of  several  hundred  people. 

In  consequence  of  the  minute  amount  of  working  material, 
Marckwald  has  not  yet  succeeded  in  purifying  the  active  sub- 
stance sufficiently  to  determine  its  spectrum. 

By  means  of  a  simple  calculation,  the  activity  of  radium  F, 
i.e.  of  radiotellurium  in  a  pure  state,  can  readily  be  deduced. 
Let  JVj  be  the  number  of  atoms  of  radium  F  in  one  gram  of 
the  radioactive  mineral,  and  N2  the  number  of  radium  atoms. 
Radium  and  radium  F  are  in  radioactive  equilibrium,  and  conse- 
quently the  same  number  of  atoms  of  each  break  up  per  second. 

Thus,  X1N1  =  XZN2, 

where  \,  A2  are  the  constants  of  change  of  radium  F  and 
radium  respectively.  Now  radium  F  is  half  transformed  in 
.38  years  and  radium  in  about  1300  years.  Consequently 


Now  it  is  probable  that  the  atomic  weights  of  radium  and 
radium  F  are  not  very  different.  Consequently,  for  every  gram 
of  radium  in  the  mineral,  there  exists  only  .29  milligram  of 
radium  F.  For  equal  weights,  the  number  of  a  particles  ex- 
pelled from  radium  F  is  3400  times  as  great  as  the  number 
expelled  from  radium  itself,  or  850  times  as  great  as  the  num- 
ber expelled  from  radium  about  one  month  old,  when  it  is  in 
equilibrium  with  its  three  rapidly  changing  a  ray  products. 

Assuming  that  the  a  particle  from  radium  F  produces  about 
the  same  amount  of  ionization  as  the  average  a  particle  from 
radium,  the  activity  of  radium  F  measured  by  the  electric 
method  should  be  850  times  greater  than  that  of  radium. 

It  has  been  found  experimentally  that  the  amount  of  radium 
in  radioactive  minerals  is  always  proportional  to  their  content 
of  uranium  and  that  for  every  gram  of  uranium  there  is  present 
3.8  x  10~7  gram  of  radium. 

The  weight  of  radium  F  per  gram  of  uranium  is  thus 
1.1  x  10~10  gram,  and  per  ton  of  2000  Ibs.,  0.1  milligram.  From 


142 


RADIOACTIVE   TRANSFORMATIONS 


15  tons  of  Joachimsthal  mineral,  which  contains  about  50  per  cent 
of  uranium,  the  yield  of  radium  F  should  be  0.75  milligrams. 

The  amount  separated  by  Marckwald  from  this  amount  of 
pitchblende  was  about  3  milligrams.  It  is  unlikely  that  the 
whole  amount  of  radium  F  was  separated,  and  the  three  milli- 
grams probably  contain  some  impurity.  The  theoretical  pro- 
portion of  radium  F  in  the  radioactive  mineral  is  thus  in  good 
agreement  with  the  experimental  results. 

Although  the  proportion  of  radium  F  in  minerals  may  appear 
extremely  small,  yet  the  other  more  rapidly  changing  products 
exist  in  still  smaller  amounts.  The  weight  of  each  product 
present  per  ton  of  uranium  is  directly  proportional  to  its  period, 
so  that  the  most  swiftly  changing  product  is  present  in  the 
smallest  quantity.  In  the  following  table  are  shown  the  weights 
of  each  of  the  radium  products  present  per  ton  of  2000  pounds 
of  uranium  in  the  mineral. 


Products. 

Period. 

Weight  in  milligrams  per  ton 
of  uranium. 

Radium 

1300  years 

340  milligrams 

Emanation 
Radium  A 

3.8  days 
3  minutes 

2.6  X  lO-3  milligrams 
1.4  X  10-6 

B 

26 

1.2  X  10-* 

C 

19 

9     X  10-6 

"        D 

40  years 

10 

E 

6  days 

4.2  X  10-3 

F 

143     " 

.1 

The  products  radium  A,  B,  C,  and  E  exist  in  far  too  small 
amounts  to  be  examined  by  ordinary  chemical  methods,  even  if 
their  short  life  allowed  it.  Radium  D,  however,  is  present  in 
considerable  quantity  compared  with  radium  F,  and  it  should 
be  possible  to  obtain  a  sufficient  amount  of  it  for  a  chemical 
examination. 

POLONIUM  AND  RADIOTELLUKIUM 

It  will  be  remembered  that  the  first  active  substance  separated 
from  pitchblende  was  found  associated  with  bismuth  and  was 
called  polonium  by  its  discoverer,  Mme.  Curie. 


SLOW   TRANSFORMATIONS   OF   RADIUM      143 

Several  methods  were  devised  for  the  concentration  of  the 
active  material  mixed  with  the  bismuth,  and  Mme.  Curie  finally 
succeeded  in  obtaining  an  active  substance  comparable  in  activ- 
ity with  radium.  The  polonium  gave  out  only  a  rays,  and  its 
activity  was  not  permanent  but  gradually  decreased. 

Both  as  regards  the  nature  of  its  rays  and  its  physical  and 
chemical  properties,  polonium  is  very  analogous  to  the  product 
radium  F  and  radiotellurium.  There  has  been  a  considerable 
amount  of  discussion  at  various  times  as  to  whether  the  active 
constituent  in  radiotellurium  is  identical  with  that  present 
in  polonium.  At  first  it  was  announced  that  the  activity  of 
radiotellurium  did  not  decay  appreciably,  and  it  apparently 
behaved  in  this  respect  quite  differently  from  polonium.  We 
now  know  that  radiotellurium  does  lose  its  activity,  and  fairly 
rapidly. 

If  the  two  products  contain  the  same  constituent,  their  ac- 
tivities should  decay  according  to  the  same  period.  Mme. 
Curie,  however,  has  observed  that  some  of  her  preparations  of 
polonium  do  not  lose  their  activity  according  to  an  exponential 
law. 

For  example,  a  sample  of  polonium  nitrate  lost  half  its 
activity  in  11  months  and  95  per  cent  in  33  months.  A  sample 
of  the  metal  lost  67  per  cent  of  its  activity  in  6  months.  These 
results  are  not  at  all  concordant.  The  sample  of  the  metal 
loses  its  activity  slightly  faster  than  radium  F,  while  the  nitrate 
at  first  loses  it  much  more  slowly.  If  these  results  are  reliable, 
the  divergence  of  the  activity  curves  from  the  exponential  law 
shows  that  more  than  one  substance  is  present  in  the  polonium 
experimented  with  by  Mme.  Curie.  It  is  very  probable  that 
this  second  constituent  is  radium  D.  The  presence  of  some  of 
this  substance,  which  gives  rise  to  radium  F,  would  cause  the 
a  ray  activity  to  decrease  at  first  more  slowly  than  the  normal 
rate  when  only  radium  F  is  present. 

Considering  the  similarity  of  polonium  and  radiotellurium  in 
their  chemical,  physical,  and  radioactive  properties  and  the 
probable  identity  of  their  periods,  I  think  that  there  can  be  no 
doubt  that  the  a  ray  constituent  in  polonium  is  the  same  as 


144  RADIOACTIVE   TRANSFORMATIONS 

that  separated  by  a  different  method  by  Marckwald.  We  may 
then  conclude  that  radiotellurium  and  polonium  are  both  de- 
rived from  the  transformation  of  the  radium  atom.1 

CONNECTION  OF  RADIOLEAD  WITH  THE  ACTIVE  DEPOSIT 

We  shall  now  describe  some  experiments  which  show  con- 
clusively that  the  product  radium  D  is  the  primary  constituent 
of  the  radiolead,  first  separated  by  Hofmann  from  pitchblende 
residues.  The  early  results  of  Hofmann,  on  the  separation  and 
properties  of  radiolead,  were  subjected  to  considerable  criticism, 
but  there  is  now  no  doubt  that  to  him  belongs  the  credit  of 
separation  of  a  new  product  from  pitchblende,  which  proves  to 
be  the  parent  of  radiotellurium  and  polonium, 

My  attention  was  first  drawn  to  the  connection  between 
radiolead  and  the  active  deposit  of  radium  by  an  examination 
of  a  specimen  of  radiolead  kindly  prepared  for  me  by  Dr.  Bolt- 
wood  of  New  Haven.  This  was  found  to  give  out  initially  an 
unusual  proportion  of  ft  rays  as  compared  with  its  a  ray  activity, 
and  the  a  ray  activity  was  found  to  increase  progressively  with 
time.  In  these  respects  it  behaved  in  a  similar  manner  to  a 
substance  initially  containing  radium  D  and  E,  in  which  the 
product  radium  F  was  being  gradually  formed,  thus  giving  rise 
to  the  increasing  a  ray  activity. 

The  connection  of  radiolead  with  radium  D,  E,  and  F  has 
been  conclusively  proved  by  a  chemical  examination  of  the 
radioactive  constituents  found  in  radiolead  and  a  determination 
of  their  periods  of  decay.  It  must  be  borne  in  mind  that  the 
name  radiolead  was  given  to  the  active  substance  because  it 
was  first  separated  mixed  with  lead,  but  we  now  know  that 
the  active  substances  contained  in  it  have  no  more  connection 
with  lead  than  radium  has  with  the  barium  from  which  it  is' 
finally  separated. 

1  The  identity  of  the  active  constituent  of  radiotellurium  and  polonium  has  now 
been  definitely  settled.  Mme.  Curie  (Comptes  rendus,  Jan.  29,  1906)  has  accurately 
determined  the  loss  of  activity  of  polonium,  and  found  that  it  decayed  according  to 
an  exponential  law  with  a  period  of  140  days.  This  period  of  decay  is  practically 
identical  with  that  found  for  radiotellurium  and  radium  F. 


SLOW   TRANSFORMATIONS    OF   RADIUM      145 

Hofmann,  Gonders,  and  Wolfl.  1  in  the  course  of  a  chemical 
examination  of  a  specimen  of  radiolead  obtained  the  following 
results.  Experiments  were  first  made  on  the  effect  of  adding 
substances  to  a  solution  of  radiolead  and  then  removing  them 
by  precipitation.  Small  quantities  of  the  platinum  metals  in 
the  form  of  chlorides  were  left  in  the  solution  for  several  weeks 
and  then  precipitated  by  formalin  or  hydroxylamine.  All  of 
these  substances  after  separation  were  found  to  give  out  a  and 
ft  rays. 

Most  of  the  ft  ray  activity  disappeared  in  about  six  weeks,  and 
the  a  ray  activity  in  about  one  year.  We  shall  see  that  the  ft  ray 
activity  is  due  to  the  separation  of  radium  E,  which  decays  to 
half  value  in  6  days,  while  the  a  ray  activity  is  due  to  radium  F. 
This  conclusion  is  further  confirmed  by  experiments  on  the 
effect  of  heat  on  the  activity  of  these  substances.  At  a  full  red 
heat,  the  a  ray  activity  was  lost  in  a  few  seconds.  This  is  in 
agreement  with  experiments  on  radium  F  which  is  volatilized 
at  about  1000°  C. 

Salts  of  gold,  silver,  and  mercury,  added  to  the  solution  of 
radiolead,  were  found  to  show  only  a  ray  activity.  This  is 
explained  if  radium  F  is  alone  removed.  Bismuth  salts,  on  the 
other  hand,  showed  a  and  ft  activity,  but  the  latter  died  away 
rapidly.  This  shows  that  bismuth  removes  both  radium  E 
and  F. 

The  a  ray  activity  of  the  radiolead  is  much  reduced  by  pre- 
cipitation of  bismuth  in  the  solution,  but  gradually  increases 
again  with  time.  This  result  is  exactly  what  is  to  be  expected 
if  radiolead  contains  radium  D,  E,  and  F.  Radium  E  and  F 
are  removed  with  the  bismuth,  while  D  is  left  behind,  and  in 
consequence  there  is  a  fresh  growth  of  radium  E  and  F. 

The  radioactive  measurements  made  by  Hofmann,  Gonders, 
and  Wolfl  were  unfortunately  not  very  precise,  but  this  want 
has  been  supplied  by  some  recent  careful  measurements  by 
Meyer  and  Schweidler.2  If  radiolead  contains  radium  D,  E, 
and  F,  the  ft  ray  activity  due  to  E  should  decay  to  half  value 

1  Hofmann,  Gonders,  and  Wolfl:  Ann.  d.  Phys .,  v,  p.  615  (1904). 

2  Meyer  and  Schweidler:  Wien  Ber.,  July,  1905. 

10 


146  RADIOACTIVE   TRANSFORMATIONS 

in  6  days,  and  the  a  ray  activity  due  to  F  to  half   value  in 
about  140  days. 

These  results  have  been  completely  confirmed  by  Meyer  and 
Schvveidler,  who  have  accurately  measured  the  rates  of  decay 
of  the  various  products  from  radiolead.  A  series  of  palladium 
plates  were  immersed  in  the  radiolead  solution.  After  removal 
the  activity  consisted  of  a  and  ft  rays.  The  /3  ray  activity 
decreased  exponentially  with  the  time,  falling  to  half  value  in 
6.2  days.  The  ft  ray  product  is  thus  identical  with  radium  E. 
The  a  ray  activity,  after  some  months,  fell  off  exponentially 
with  a  period  of  135  days.  The  a  ray  product  is  thus  identical 
with  radium  F. 

There  is  thus  no  doubt  that  radiolead  some  time  after  its 
preparation  contains  radium  D,  E,  and  F.  No  observations 
have  so  far  been  made  to  settle  definitely  whether  radium  D,  E, 
and  F  are  removed  together  with  the  lead  or  whether  only  ra- 
dium D  is  removed  and  the  presence  of  radium  E  and  F  after 
some  time  is  due  to  their  production  from  radium  D.  If  the 
bismuth  is  separated  from  the  lead,  it  seems  likely  that  radium  E 
and  F  would  be  removed  with  the  former  and  that  radium  D 
alone  would  be  removed  with  the  lead. 

It  is  thus  seen  that  the  primary  constituent  in  radiolead  is 
the  parent  of  radiotellurium  and  polonium. 

The  connection  of  the  radium  products  with  radiolead  is  out- 
lined in  the  following  table. 

Radium  D  =  product  in  new  radiolead.  No  rays.  Half  transformed 
in  40  years. 

Radium  E  gives  out  ft  rays ;  is  separated  with  bismuth,  iridium, 
and  palladium.  Half  transformed  in  6  days. 

Radium  F  =  product  in  polonium  and  radiotellurium.  Gives  out 
only  a  rays.  Volatile  at  1000°  C.,  and  attaches  it- 
self to  bismuth  and  palladium.  Half  transformed 
in  143  days. 

These  results  have  thus  emphasized  the  importance  of  radium 
D  as  a  new  radioactive  substance  in  pitchblende. 


SLOW   TRANSFORMATIONS    OF  RADIUM       147 

It  has  been  shown  that  about  10  milligrams  of  radium  D 
should  be  separated  from  the  mineral  for  each  ton  of  uranium 
present.  A  few  weeks  after  separation  the  /8  ray  activity  of 
this  substance  should  be  about  30  times  as  great  as  that  of 
radium.  A  quantity  of  this  substance  would  serve  as  a  useful 
source  of  /3  rays,  and  also  as  a  very  convenient  means  of  obtain- 
ing radium  F.  A  very  active  deposit  of  this  substance  could 
at  any  time  be  obtained  by  placing  a  bismuth  or  palladium  plate 
in  the  solution.  It  is  to  be  hoped  that  this  substance  will  be 
separated  from  pitchblende  residues  at  the  same  time  as  radium, 
for  in  many  respects  it  would  prove  as  useful  in  experiments  as 
radium  itself. 


CHAPTER  VI 
ORIGIN   AND   LIFE   OF   RADIUM 

Since  radium  itself  continuously  throws  off  a  particles  and 
gives  rise  to  a  radioactive  gas,  its  amount  must  steadily  de- 
crease with  time.  Radium,  in  this  respect,  must  be  considered 
as  a  radioactive  product  like  the  emanation,  the  only  difference 
being  its  comparatively  slow  rate  of  change.  A  given  amount 
of  radium  left  to  itself  must  ultimately  disappear  as  such,  and 
after  a  series  of  transformations  there  will  only  remain  the 
inactive  substances  produced  by  its  decomposition. 

The  time  of  observation  has  been  far  too  short  to  fix  the 
period  of  change  of  radium  by  direct  experiment.  Accurate 
measurements  of  the  activity  will  not  supply  any  information 
of  value  on  this  point  for  a  long  interval,  since  the  slow  trans- 
formation products  arising  from  the  radium  actually  cause  a 
steady  increase  of  activity  for  several  hundred  years. 

There  are  several  indirect  methods  which  can  be  employed 
to  deduce  the  probable  period  of  radium,  depending  on  (1)  the 
number  of  a  particles  expelled  per  second,  (2)  the  observed 
heating  effect  of  radium,  and  (3)  the  observed  volume  of  the 
emanation  released  from  it. 

Method  1.  We  shall  first  consider  the  method  based  on  the 
rate  of  expulsion  of  a  particles.  By  measuring  the  charge 
carried  by  the  a  rays  expelled  from  a  thin  film  of  radium,  the 
writer 1  found  that  the  total  number  of  a  particles  expelled  per 
second  from  one  gram  of  radium  at  its  minimum  activity  was 
6.2  x  1010,  assuming  that  each  a  particle  carries  with  it  the 
usual  ionic  charge  of  3.4  xlO~10  electrostatic  units.  When  in 
radioactive  equilibrium  with  its  family  of  rapidly  changing 

1  Rutherford:  Phil.  Mag.,  Aug.,  1905. 


ORIGIN  AND   LIFE   OF  RADIUM  149 

products,  the   number  of   expelled   particles    is   four  times  as. 
great. 

The  simplest  assumption  is  that  one  a  particle  is  expelled 
from  each  atom  as  it  breaks  up.  Consequently  6.2  x  101(> 
atoms  of  radium  break  up  per  second.  Now  it  has  been  found 
from  data  based  upon  experiment  that  one  cubic  centimetre  of  a 
gas,  —  hydrogen,  for  example,  —  at  standard  pressure  and  tem- 
perature contains  3.6  x  1019  molecules.  From  this  it  follows 
that  one  gram  of  radium  of  atomic  weight  225  contains 
3.6xl021  atoms  of  radium.  The  fraction  of  radium  which 
breaks  up  per  second  is 

6.2x10" 
3.6  X  1021  - 

or  5.4  x  lO"4  per  year. 

Like  any  other  active  product,  the  amount  of  radium  must 
decrease  according  to  an  exponential  law,  so  that  the  value  of 
its  constant  of  change  \  is  5.4  x  10"4  (year)"1. 

From  this  it  follows  that  half  of  the  radium  is  transformed  in 
about  1300  years.  The  average  life  of  the  radium  atom  which 
is  measured  by  I/A,  is  about  1800  years. 

Method  2.  The  calculation  of  the  life  of  radium  can  also  be 
based  on  the  observed  heating  effect  of  radium,  which  will  be 
shown  later  (Chapter  X)  to  be  a  direct  measure  of  the  kinetic 
energy  of  the  expelled  a  particles.  From  measurements  of  the 
velocity  and  mass  of  the  a  particle  expelled  from  radium,  the 
average  energy  of  motion,  £  mv2,  of  the  a  particle  was  found  by 
the  writer 'to  be  5.9  x  10"6  ergs.  Now  it  is  found  experi- 
mentally that  one  gram  of  radium  emits  heat  at  the  rate  of 
about  100  gram  calories  per  hour.  If  this  is  due  to  the  kinetic 
energy  of  the  a  particles,  the  number  of  such  particles  that 
must  be  expelled  per  second  is  about  2.0  x  10n.  The  number 
from  radium  itself  is  one  quarter  of  this.  Using  the  same 
method  of  calculation  as  before,  it  is  seen  that  half  of  the 
radium  is  transformed  in  about  1600  years  —  a  value  not  very 
different  from  that  deduced  by  the  first  method. 

Method  3.    We  shall  now  consider  the   calculation   of  the 


150  RADIOACTIVE   TRANSFORMATIONS 

life  of  radium  based  on  the  observed  volume  of  the  emanation 
released  from  one  gram  of  radium.  Ramsay  and  Soddy  found 
that  this  maximum  volume  was  slightly  greater  than  one  cubic 
millimetre  at  standard  pressure  and  temperature.  Now  one 
cubic  millimetre  of  gas  contains  3.6  x  1019  molecules.  The 
number  of  molecules  of  emanation  produced  per  second  is  X 
times  the  equilibrium  number  present,  where  X  is  the  constant 
of  change  of  the  emanation.  Assuming,  as  is  probably  the 
case,  that  the  emanation  is  a  monatomic  gas,  and  that  each 
atom  of  radium  in  breaking  up  gives  rise  to  one  atom  of  emana- 
tion, the  number  of  atoms  of  radium  breaking  up  per  second  is 
7.6  x  1010.  Proceeding  as  before,  this  gives  1050  years  as  the 
period  of  radium. 

The  first  two  methods  involve  the  assumption  of  the  number 
of  atoms  present  in  one  cubic  centimetre  of  a  gas.  The  calcu- 
lation based  on  the  volume  of  the  emanation  can,  however,  be 
made  in  a  different  way  without  this  assumption.  If  one  atom 
of  radium  by  the  loss  of  one  a  particle  is  changed  into  one  atom 
of  emanation,  the  molecular  weight  of  the  latter  must  be  at 
least  200.  The  value  deduced  from  experiments  on  diffusion 
is  about  100,  but  on  page  85  some  reasons  have  been  given  in 
support  of  the  view  that  this  is  an  underestimate.  One  cubic 
millimetre  of  the  emanation  thus  weighs  as  much  as  100  c.mms. 
of  hydrogen,  i.  e.  8.96  x  10"6  grams.  The  weight  of  emanation 
produced  per  second  is  X  times  this  amount,  i.  e.  1.9  x  10~u 
grams.  The  weight  of  emanation  produced  per  year  is  thus 
6  x  10"4  grams,  and  this  must  be  nearly  equal  to  the  weight  of 
radium  breaking  up  per  year.  This  makes  the  period  of  radium 
about  1300  years. 

Considering  the  uncertainty  attaching  to  the  exact  values  of 
the  data  used  in  these  calculations,  the  periods  deduced  by  the 
three  methods  are  in  good  agreement.  In  calculations  we  shall 
take  1300  years  as  the  most  probable  value  of  the  period  of 
radium. 

Radium  thus  breaks  up  at  a  fairly  rapid  rate,  and  in  the 
course  of  a  few  thousand  years  a  mass  of  radium  left  by  itself 


ORIGIN  AND   LIFE    OF   RADIUM  151 

would  lose  a  large  proportion  of  its  activity.  Assuming  that 
radium  breaks  up  with  a  period  of  1300  years,  it  can  readily  be 
calculated  that  after  an  interval  of  26,000  years,  only  one  mil- 
lionth of  the  mass  of  radium  would  remain  unchanged.  If  we 
suppose,  for  illustration,  that  the  earth  was  originally  composed 
of  pure  radium,  the  activity  observed  in  the  earth  26,000  years 
later  would  be  about  the  same  as  that  observed  to-day  in  a  good 
specimen  of  pitchblende.  This  period  of  years  is  very  small 
compared  with  the  age  of  the  minerals  of  the  earth,  and  unless 
the  very  improbable  assumption  is  made,  that  the  radium  was 
in  some  way  suddenly  formed  at  a  very  late  period  in  the 
earth's  history,  we  are  forced  to  the  conclusion  that  radium 
must  be  continuously  produced  in  the  earth.  It  was  early  sug- 
gested by  Rutherford  and  Soddy  that  radium  might  be  a  disinte- 
gration product  of  one  of  the  radioactive  elements  present  in 
pitchblende.  Both  uranium  and  thorium  fulfil  the  conditions 
required  as  a  possible  parent  of  radium.  Both  have  atomic 
weights  greater  than  that  of  radium,  and  both  are  transformed  at 
a  very  slow  rate  compared  with  radium.  A  cursory  examination 
shows  that  uranium  is  the  most  likely  parent,  since  radium* is 
always  found  in  largest  amount  in  uranium  minerals,  while  some 
thorium  minerals  contain  very  little  radium. 

We  shall  now  consider  some  of  the  consequences  that  should 
follow  if  uranium  is  considered  to  be  the  parent  of  radium. 

Several  thousand  years  after  the  uranium  has  been  formed,  the 
amount  of  radium  should  reach  a  definite  maximum  value.  Its 
rate  of  production  by  the  uranium  is  then  balanced  by  its  own 
rate  of  disappearance.  Under  such  conditions,  the  number  of 
atoms  of  radium  which  break  up  per  second  is  equal  to  the 
number  of  atoms  of  uranium  which  break  up  per  second.  Now, 
as  far  as  observation  has  gone,  uranium  only  emits  one  a  par- 
ticle during  its  transformation  into  uranium  X.  The  product 
uranium  X  does  not  emit  a  rays,  but  only  /3  and  7  rays.  On 
the  other  hand,  we  have  seen  that  radium  itself  and  four  of  its 
products,  viz.,  the  emanation,  radium  A,  C,  and  F,  emit  a  rays. 
The  number  of  a  particles  expelled  from  the  radium,  and  these 
products  of  its  transformation,  should  thus  be  five  times  the 


152  RADIOACTIVE   TRANSFORMATIONS 

number  expelled  from  uranium.  Assuming  that  the  a  particles 
from  the  radium  products  produce  about  the  same  ionization  as 
the  a  particle  from  uranium,  the  activity  of  a  radioactive  mineral, 
which  consists  mostly  of  uranium,  should  be  about  six  times  that 
of  uranium  itself.  Now  the  best  pitchblende  shows  an  activ- 
ity about  five  times  that  of  uranium,  so  that  the  theoretical 
result  is  approximately  realized  in  practice.  Until,  however,  the 
relative  ionizations  produced  by  the  a  particles  from  uranium 
and  each  of  the  radium  products  are  accurately  known,  the 
relative  activities  to  be  expected  cannot  be  fixed  with  certainty. 

Another  consequence  of  the  theory  is  that  the  amount  of 
radium  in  any  radioactive  mineral  should  be  always  proportional 
to  its  content  of  uranium.  This  must  hold  in  every  case,  pro- 
vided neither  the  uranium  nor  the  radium  have  been  removed 
from  the  mineral  by  physical  or  chemical  action.  This  interest- 
ing question  has  been  experimentally  attacked  by  Boltwood,1 
Strutt,2  and  McCoy,3  and  has  yielded  results  of  the  highest 
importance. 

McCoy  accurately  compared  the  activities  of  different  radio- 
active minerals,  and  showed  that  in  every  case  the  activity  was 
very  nearly  proportional  to  their  percentage  content  of  uranium. 
Since,  howejfc",  the  radioactive  minerals  contain  some  actinium, 
and  occasionally  some  thorium,  these  results  indicate  that  the 
activity  of  all  these  substances,  taken  together,  is  proportional 
to  the  amount  of  uranium.  Boltwood  and  Strutt  employed  a 
more  direct  method,  by  determining  the  relative  content  of 
uranium  and  radium  in  radioactive  minerals.  The  amount  of 
uranium  was  determined  by  direct  chemical  analysis,  while  the 
amount  of  radium  was  determined  by  measurements  of  the 
amount  of  radium  emanation  released  by  solution  of  the  mineral. 
The  relative  amount  of  the  latter  can  be  determined  with  great 
accuracy  by  the  electric  method,  which  is  the  most  convenient 
method  of  comparing  quantitatively  the  amounts  of  radium  in 
different  minerals. 

1  Boltwood:  Nature,  May  25,  1904;  Phil.  Mag.,  April,  1905. 

2  Strutt:  Trans.  Roy.  Soc.  A.,  1905. 

8  McCoy :  Ber.  d.  d.  chem.  Ges.,  No.  11,  p.  2641,  1904. 


ORIGIN   AND   LIFE   OF   RADIUM 


153 


The  results  of  both  these  observers  show  that  there  is  a  nearly 
constant  ratio  between  the  amounts  of  radium  and  uranium  in 
every  mineral  examined,  except  in  one  case,  which  will  be  con- 
sidered later.  Minerals  were  obtained  from  various  localities, 
both  in  Europe  and  America,  which  varied  widely  in  chemical 
composition  and  in  the  percentage  content  of  uranium.  The 
experiments  of  Dr.  Boltwood  of  Yale  University,  which  have 
been  made  with  great  care  and  accuracy,  show  a  surprisingly 
constant  ratio  between  the  amounts  of  uranium  and  radium. 

A  brief  account  will  be  given  of  the  methods  employed  by 
him  in  his  measure- 
ments. The  percentage 
of  uranium  in  the  min- 
eral under  consideration 
was  first  determined  by 
chemical  analysis.  A 
known  weight  of  the 
finely  powdered  mineral 
was  placed  in  a  glass 
vessel  A  (Fig.  39),  and 
sufficient  acid  intro- 
duced to  dissolve  it. 

The  acid  was  then 
boiled  until  the  mineral 
was  completely  dis- 
solved, and  the  emana- 
tion mixed  with  air  was  collected  on  the  top  of  the  column  of 
water  in  the  tube  D.  This  emanation  was  then  introduced  into 
a  closed  electroscope  of  the  type  shown  in  Fig.  6,  page  29,  which 
was  first  exhausted.  Air  was  then  introduced  until  the  gas 
inside  the  electroscope  was  at  atmospheric  pressure.  On  ac- 
count of  the  excited  activity  produced  by  the  emanation,  the 
rate  of  discharge  of  the  electroscope  did  not  reach  a  maximum 
until  about  three  hours  after  the  introduction  of  the  emana- 
tion. The  rate  of  movement  of  the  gold  leaf  of  the  elec- 
troscope was  taken  as  a  measure  of  the  amount  of  emanation 
present.  The  emanations  of  thorium  or  actinium,  released 


FIG. 


154  RADIOACTIVE   TRANSFORMATIONS 

from  the  mineral  at  the  same  time  as  the  radium  emanation, 
had,  on  account  of  the  rapid  decay  of  their  activity,  completely 
disappeared  before  the  introduction  of  the  radium  emanation 
into  the  electroscope.  This  process  was  repeated  for  all  the 
minerals  examined. 

Boltwood  observed  that  some  minerals  had  considerable  eman- 
ating power,  i.  e.,  the  minerals  lost  some  of  their  emanation 
when  in  the  solid  state.  Under  these  conditions,  the  amount 
of  emanation  released  by  solution  and  boiling  of  the  mineral 
would  be  less  than  the  equilibrium  amount.  The  proper  correc- 
tion was  made  by  sealing  up  a  known  weight  of  the  mineral  in  a 
tube  for  one  month  and  then  measuring  with  the  same  electro- 
scope the  amount  of  emanation  which  collected  in  the  air  above 
the  mineral.  The  sum  of  the  two  amounts  gives  the  true 
equilibrium  quantity  of  emanation  corresponding  to  the  radium 
present  in  the  mineral. 

The  results  obtained  by  Boltwood  are  shown  in  the  following 
table.  The  numbers  in  column  I  give,  in  arbitrary  units,  the 
amount  of  emanation  released  by  solution  and  boiling;  column  II 
shows  the  percentage  of  the  emanation  which  escaped  into  the 
air ;  column  III  shows  the  amount  of  uranium  in  the  mineral ; 
and  column  Jjt  the  numbers  obtained  by  dividing  the  equilib- 
rium amoun^m  emanation  by  the  quantity  of  uranium  present. 

If  the  amount  of  radium  always  bears  a  definite  ratio  to  the 
amount  of  uranium,  the  numbers  in  column  IV  should  be  the 
same.  With  the  exception  of  some  of  the  monazites,  there  is 
a  remarkably  good  agreement,  and,  taking  into  consideration 
the  great  variation  in  the  amount  of  uranium  in  the  different 
minerals,  and  the  wide  range  of  locality  from  which  they  were 
obtained,  the  results  afford  a  direct  and  satisfactory  proof  that 
the  amount  of  radium  in  minerals  is  directly  proportional  to  the 
amount  of  uranium  present. 

As  an  example  of  the  confidence  to  be  placed  in  this  ratio  as  a 
physical  constant  for  all  radioactive  minerals,  Boltwood  observed 
that  some  of  the  monazites  contained  a  considerable  quantity  of 
radium,  although  the  previous  analyses  had  not  shown  any 
uranium  to  be  present.  A  careful  examination  was  made  to 


ORIGIN  AND   LIFE   OF  RADIUM 


155 


Substance. 

Locality. 

I. 

II. 

in. 

IV. 

Uraninite 

North  Carolina 

170.0 

11.3 

0.7465 

228 

Uraninite 

Colorado 

155.1 

5.2 

06961 

223 

Gummite 

North  Carolina 

147.0 

13.7 

0.6538 

225 

Uraninite 

Joachimsthal 

139.6 

5.6 

0.6174 

226 

Uranophane 

North  Carolina 

117.7 

8.2 

0.5168 

228 

Uraninite 

Saxony 

115.6 

2.7 

0.5064 

228 

Uranophane 

North  Carolina 

113.5 

22.8 

0.4984 

228 

Thorogummite 

North  Carolina 

72.9 

16.2 

0.3317 

220 

Carnotite 

Colorado 

49.7 

16.3 

0.2261 

220 

Uranothorite 

Norway 

25.2 

1.3 

0.1138 

221 

Samarskite 

North  Carolina 

23.4 

0.7 

0.1044 

224 

Orangite 

Norway 

23.1 

1.1 

0.1034 

223 

Euxinite 

Norway 

19.9 

0.5 

0.0871 

228 

Thorite 

Norway 

16.6 

6.2 

0.0754 

220 

Fergusonite 

Norway 

12.0 

0.5 

0.0557 

215 

Aeschynite 
Xenotine 

Norway 
Norway 

10.0 
1.54 

0.2 

26.0 

0.0452 
0.0070 

221 
220 

Monazite  (sand) 

North  Carolina 

0.88 

0.0043 

205 

Monazite  (crys.) 

Norway 

0.84 

1.2 

0.0041 

207 

Monazite  (sand) 

Brazil 

0.76 

. 

0.0031 

245 

Monazite  (massive) 

Connecticut 

0.63 

0.0030 

210 

test  this  point,  and  it  was  found  that  uranium  was  present  in 
the  amount  to  be  expected  according  to  theory.  The  failure  to 
detect  the  presence  of  uranium  in  the  earlier  analysis  was  due 
to  the  presence  of  phosphates.  « 

There  is  one  interesting  apparent  exception  to  this  constancy 
of  the  ratio  between  the  amounts  of  uranium  and  radium. 
Danne  recently  found  that  considerable  quantities  of  radium 
are  present  in  certain  deposits  in  the  neighborhood  of  Issy 
1'Eveque  in  the  Saone-et-Loire  district,  but  that  no  trace  of 
uranium  could  be  detected.  The  active  matter  is  found  in 
pyromorphite  (phosphate  of  lead),  and  in  clays  containing 
lead,  but  the  radium  is  usually  found  in  greater  quantities  in 
the  former.  The  pyromorphite  is  found  in  veins  of  quartz  and 
felspar  rocks.  The  veins  were  always  wet,  owing  to  the  pres- 
ence of  springs  in  the  neighborhood.  The  percentage  of  uranium 
in  the  pyromorphite  varies  considerably  for  different  specimens, 
but  Danne  states  that  on  an  average  one  centigram  of  radium 
is  present  per  ton. 

It  seems  probable  that  this  radium  has  been  deposited  in  the 
rocks  after  being  carried  from  a  distance  by  means  of  under- 


156  RADIOACTIVE   TRANSFORMATIONS 

ground  springs.  The  presence  of  radium  in  this  district  is  not 
surprising,  for  crystals  of  autunite  have  been  found  about  forty 
miles  distant.  This  result  is  of  interest,  for  it  suggests  that 
radium  can  in  some  cases  be  removed  from  the  radioactive 
mineral  by  solution  in  water,  and  be  deposited  under  suitable 
physical  and  chemical  conditions  some  distance  away.  It  also 
suggests  the  possibility  that  deposits  containing  a  considerable 
proportion  of  radium  may  yet  be  discovered  in  positions  where 
the  conditions  necessary  for  the  solution  and  re-deposit  of  the 
radium  are  favorable. 

AMOUNT  OF  RADIUM  IN  MINERALS 

The  weight  of  radium  in  a  mineral,  per  gram  of  uranium,  is 
thus  a  definite  constant  of  considerable  practical  as  well  as 
theoretical  importance.  This  constant  was  recently  determined 
by  Rutherford  and  Boltwood  by  comparison  of  the  amount  of 
emanation  liberated  from  a  known  weight  of  uraninite  with 
that  released  from  a  known  quantity  of  pure  radium  bromide 
in  solution.  For  the  latter  purpose,  a  known  weight  of  radium 
bromide  was  taken  from  a  sample  of  radium  bromide  obtained 
from  the  Quinin  Fabrik,  Braunschweig,  which  had  previously 
been  found  to  give  out  heat  at  a  rate  of  over  100  gram  calories 
per  hour.  P.  Curie  and  Laborde  found  that  their  pure  radium 
chloride  preparations  gave  out  heat  at  the  rate  of  about  100  gram 
calories  per  hour.  We  may  thus  conclude  that  the  radium  prep- 
aration employed  was  nearly  pure.  This  known  weight  —  about 
one  milligram  —  was  dissolved  in  water,  and  by  successive  dilu- 
tions a  standard  solution  was  made  up  containing  10"6  grams  of 
radium  bromide  per  cubic  centimetre.  Taking  the  constitution 
of  radium  bromide  as  RaBr2  and  the  atomic  weight  of  radium  as 
225,  it  was  deduced  that  in  each  gram  of  uranium  in  the  mineral 
the  corresponding  weight  of  radium  was  3.8  x  10~7  gram.1 

From  this  it  follows  that  .34  gram  of  radium  is  present  in 
a  mineral  per  ton  of  uranium.  Since  the  radioactive  minerals 

1  The  first  determination  of  this  constant  by  Rutherford  and  Boltwood  (Amer. 
Journ.  Sci.,  July,  1905)  gave  a  value  of  7.4  X  10~7.  This  was  later  found  by  them 
to  be  incorrect,  owing  to  a  precipitation  of  the  radium  in  the  standard  solution. 


ORIGIN  AND   LIFE   OF  RADIUM  157 

from  which  radium  is  extracted  usually  contain  about  50  per 
cent  of  uranium,  the  yield  of  radium  per  ton  of  mineral  should 
be  about  .17  gram. 

Assuming,  as  a  first  approximation,  that  the  a  particles  from 
radium  and  its  products,  and  from  uranium,  are  expelled  at  the 
same  speed,  the  activity  of  the  radium  and  its  family  of  rapidly 
changing  products  when  in  equilibrium  with  the  uranium, 
should  be  four  times  that  of  uranium.  Taking  the  activity  of 
pure  radium  as  about  three  million  times  that  of  uranium,  the 
weight  of  radium  required  to  produce  this  activity  is 

A 

—  1.33  X  10-6  grams. 


3  x  106 

The  observed  amount,  3.8  x  10~7  grams,  is  considerably 
smaller.  The  agreement  between  theory  and  experiment,  how- 
ever, becomes  much  closer  when  we  take  into  account  the  known 
fact  that  the  average  a  particle  from  radium  has  a  greater  pene- 
trating power  and  consequently  produces  a  greater  number  of  ions 
in  the  gas  than  the  average  a  particle  expelled  from  uranium. 

GROWTH  OF  RADIUM  IN  URANIUM  SOLUTIONS 

Although  the  constancy  of  the  ratio  between  the  amounts  of 
radium  and  uranium  in  all  radioactive  minerals,  as  well  as  the 
agreement  between  the  theoretical  and  observed  quantities,  afford 
very  strong  proof  of  the  truth  of  the  theory  that  uranium  is  the 
parent  of  radium,  yet  this  conclusion  cannot  be  considered  as 
completely  established  until  it  has  been  experimentally  shown 
that  radium  gradually  collects  in  uranium  solutions  originally 
freed  from  it. 

The  rate  of  production  of  radium  on  the  disintegration  theory 
can  readily  be  estimated.  The  fraction  of  radium  breaking  up 
per  year  has  been  calculated  on  page  149  and  shown  to  be  about 
5.4  x  ID"4  per  year.  The  amount  of  radium  per  gram  of 
uranium  in  minerals  has  been  shown  to  be  3.8  x  10~7  grams. 
Consequently,  in  order  to  keep  up  the  quantity  of  radium  in  a 
mineral  at  a  constant  amount,  the  rate  of  supply  per  year  per 
gram  of  uranium  must  be  5.4  x  1Q-4  x  3.8  x  10~7  =  2  x  1Q-10 


158  RADIOACTIVE   TRANSFORMATIONS 

gram.  This  represents  the  amount  of  radium  formed  per  year 
from  each  gram  of  uranium.  The  presence  of  radium  can  readily 
be  detected  by  its  emanation.  Using  a  kilogram  of  uranium, 
the  amount  of  radium  formed  per  year  is  2  x  10~7  gram.  The 
emanation  from  this  would  cause  a  gold-leaf  electroscope  to  be 
discharged  in  a  few  seconds,  while  the  amount  of  radium  pro- 
duced in  a  single  day  should  be  easily  measurable. 

Experiments  on  the  growth  of  radium  in  uranium  were  first 
undertaken  by  Soddy.1  A  kilogram  of  uranium  nitrate  in 
solution  was  employed.  This  was  first  chemically  treated,  to 
remove  most  of  the  small  quantity  of  radium  originally  present, 
and  was  then  allowed  to  stand  in  a  closed  vessel.  The  equilib- 
rium amount  of  emanation  formed  in  the  solution  was  then 
tested  at  intervals.  Preliminary  experiments  showed  that  the 
rate  of  production  of  the  radium  was  certainly  far  slower  than 
the  theoretical  value,  and  at  first  little  if  any  indication  of  pro- 
duction of  radium  was  observed.  In  later  experiments,  how- 
ever, Soddy  found  that  in  the  course  of  eighteen  months,  the 
amount  of  radium  in  the  solution  had  distinctly  increased. 

The  solution  after  this  interval  contained  about  1.6  x  10~9 
gram  of  radium.  This  gives  the  value  of  about  2  x  10~12  as 
the  fraction  of  uranium  changing  per  year,  while  the  theoreti- 
cal fraction  is  2  x  10~10,  or  100  times  greater  than  the  observed 
amount. 

Whetham  also  found  a  similar  result,  but  concluded  that  the 
rate  of  production  was  faster  than  that  observed  by  Soddy.  On 
the  other  hand,  Boltwood  finds  no  certain  evidence  of  the  growth 
of  radium  from  uranium,  although  an  extremely  minute  quan- 
tity was  detectable  in  his  apparatus.  In  his  experiments,  100 
grams  of  uranium  were  obtained  almost  completely  free  from 
radium  by  fractional  crystallization.  After  this  treatment,  no 
trace  of  radium  could  be  detected  in  his  uranium  solution, 
although  he  could  with  certainty  have  detected  the  presence  of 
1.7  X  10"11  grams. 

After  standing  for  a  year,  no  effect  was  produced  by  the 
emanation  in  his  electroscope,  which  was  of  the  same  sensitive- 

1  Soddy:  Nature,  May  12,  1904,  Jan.  19,  1905;  Phil.  Mag.,  June,  1905. 


ORIGIN  AND   LIFE   OF  RADIUM 

ness  as  in  the  first  experiments.  Such  a  result  shows  that 
uranium,  when  purified  in  the  manner  adopted  by  Boltwood, 
certainly  does  not,  in  the  course  of  a  year,  grow  a  measurable 
quantity  of  .radium,  and  that  the  quantity  is  not  more  than  one 
thousandth  of  the  theoretical  amount. 

Although  the  experimental  evidence  is  somewhat  conflicting, 
I  think  there  can  be  little  doubt  that  the  uranium  of  Soddy  did 
show  a  growth  of  radium,  although  only  a  fraction  of  the 
amount  to  be  expected  theoretically.  So  far  as  is  known  at 
present,  uranium  breaks  up  with  the  expulsion  of  an  a  particle 
and  produces  uranium  X,  which  has  a  period  of  22  days  and 
emits  only  ft  and  7  rays.  No  further  active  product  has  been 
detected,  so  that  we  are  unable  to  say  what  further  stages  of 
disintegration  appear  before  radium  is  formed.  If,  for  example, 
the  disintegration  product  of  UrX  is  a  rayless  substance  with 
a  very  slow  period,  the  slow  rate  of  production  of  radium  by 
uranium  is  at  once  explained.  Suppose,  for -example,  that  the 
uranium,  as  in  the  experiments  of  Boltwood,  was  carefully  puri- 
fied. It  is  probable  that  the  rayless  product  would  be  com- 
pletely removed  from  the  uranium.  Before  radium  could  be 
produced  at  an  appreciable  rate,  the  intermediate  rayless  prod- 
uct must  be  formed  in  some  quantity.  If  the  rayless  product 
had  a  period  of  several  thousand  years,  an  interval  of  several 
years  would  be  required  before  the  appearance  of  radium  could 
be  detected. 

Such  an  hypothesis  of  an  intermediate  transition  product 
would  also  account  for  the  discrepancy  between  the  experi- 
ments of  Soddy  and  Boltwood.  In  the  experiments  of  the 
former,  the  trace  of  radium  initially  observed  in  the  uranium 
was  partly  removed  by  the  precipitation  of  barium  in  the 
uranium  solution.  This  may  not  have  removed  the  inter- 
mediate product  which  had  been  collecting  in  the  uranium  for 
several  years.  Consequently,  the  unpurified  solution  used  by 
Soddy  was  better  suited  to  show  the  production  of  radium  than 
the  carefully  treated  solution  used  by  Boltwood. 

I  think  that  there  can  be  no  reasonable  doubt  that  the  pure 
uranium  solution  will  ultimately  show  the  presence  of  radium, 


160  RADIOACTIVE   TRANSFORMATIONS 

although  an  interval^  of  several  years  may  be  required  before  the 
amount  formed  is  detectable. 

The  changes  occurring  in  uranium  which  lead  to  the  pro- 
duction of  radium  are  shown  below. 

Uranium. 

Uranium  X. 

1 

One  or  more  unknown  transition  substances  with  long 

periods  of  transformation. 

f 

Kadiuin  and  its  family  of  products. 

There  can  be  little  doubt  that  the  intermediary  product  or 
products  between  uranium  X  and  radium  will  ultimately  be  sepa- 
rated chemically.  Supposing  that  there  is  only  one  intermediary 
product,  it  is  not  unlikely  that  this  will  prove  to  be  rayless  in 
character.  The  presence  of  such  a  product  could  be  detected 
by  its  property  of  producing  radium  initially  at  a  constant  rate. 
If,  for  example,  the  unknown  product  were  completely  separated 
from  an  amount  of  radioactive  mineral  which  contained  a  kilo- 
gram of  uranium,  it  would  produce  radium  initially  at  the  rate 
of  about  4  x  10~7  gram  per  year,  or  10~9  of  a  gram  per  day. 
This  latter  amount  is  easily  measurable,  and  consequently  a 
proof  of  the  production  of  radium  by  this  substance  should  only 
require  observations  extending  over  a  few  weeks. 

The  position  that  radium  holds  in  regard  to  uranium  is  unique 
in  chemistry.  For  the  first  time  it  is  possible  to  predict  accu- 
rately the  amount  of  one  element  present  when  the  quantity  of 
another  is  known.  In  seems  probable  that  such  relations  will 
ultimately  be  extended  to  include  all  the  radioelements  and 
their  products,  and  possibly  also  some  of  the  apparently  non- 
radioactive  substances;  for  it  is  remarkable  how  certain  ele- 
ments are  always  found  together  in  mineral  deposits  in  about 
the  same  relative  amounts,  although  there  is  no  apparent  chem- 
ical reason  for  their  association. 


CHAPTER  VII 

TRANSFORMATION  PRODUCTS  OF  URANIUM   AND  ACTINIUM, 
AND  THE  CONNECTION  BETWEEN  THE  RADIOELEMENTS 

We  have  in  previous  chapters  analyzed  in  some  detail  the 
series  of  transformations  that  take  place  in  thorium  and  radium. 
As  the  two  other  radioactive  substances,  uranium  and  actinium, 
are  also  of  interest  in  this  connection,  a  brief  review  will  now 
be  given  of  the  changes  taking  place  in  them. 

CHANGES  IN  URANIUM 

Uranium  products  give  out  a,  /3,  and  7  rays,  but  no  definite 
evidence  has  yet  been  obtained  that  uranium  gives  off  an  ema- 
nation. In  this  respect,  it  appears  to  differ  from  thorium, 
radium,  and  actinium.  It  is,  however,  possible  that  a  closer 
investigation  may  yet  disclose  the  presence  of  an  emanation 
with  a  very  short  life.  If  an  emanation  were  emitted  which 
lasted  for  less  than  a  hundredth  part  of  a  second,  its  detection 
by  the  electric  method  would  be  extremely  difficult. 

Only  one  direct  transformation  product,  called  uranium  X,  has 
so  far  been  observed  in  uranium.  The  separation  of  this  sub- 
stance was  first  effected  by  Sir  William  Crookes 1  by  two  dis- 
tinct methods.  Ammonium  carbonate  in  excess  was  added  to 
a  uranium  solution  and  the  uranium  precipitated.  A  light 
precipitate  remained  behind  which  contained  the  UrX.  Crookes 
used  the  photographic  method  and  observed  that  the  uranium, 
after  this  treatment,  was  photographically  almost  inactive, 
while  the  precipitate  containing  the  UrX,  when  compared  with 
an  equal  weight  of  uranium,  had  a  very  intense  photographic 
action.  The  explanation  of  this  was  made  clear  by  later  ex- 
periments. UrX  gives  out  only  /3  rays,  which,  in  the  case  of 
uranium,  produce  far  more  photographic  action  than  the  easily 

1  Crookes:  Proc.  Roy.  Soc.,  Ixvi,  p.  409  (1900). 
11 


162  RADIOACTIVE   TRANSFORMATIONS 

absorbed  a  rays.  The  removal  of  UrX  does  not  in  any  way 
alter  the  a  ray  activity  of  uranium,  measured  by  the  electric 
method,  but  completely  removes  the  /3  ray  activity. 

The  second  method  used  by  Crookes  was  to  dissolve  uranium 
in  ether,  when  the  uranium  divides  itself  unequally  between 
the  ether  and  water  present.  The  water  fraction  contains  all 
the  UrX,  while  the  ether  fraction  is  photographically  inactive. 

Still  another  means  of  separation  of  UrX  was  used  by  Bec- 
querel.1  A  small  quantity  of  a  barium  salt  was  added  to  a 
uranium  solution,  and  then  precipitated  by  the  addition  of  sul- 
phuric acid.  The  dense  barium  precipitate  carries  down  the 
UrX  with  it,  and,  after  several  successive  treatments,  the  UrX 
is  almost  completely  removed  from  the  uranium.  Becquerel 
first  noted  that  the  UrX  loses  its  activity  after  some  time,  while 
the  uranium  recovers  its  lost  activity. 

The  rate  at  which  UrX  loses  its  activity  was  determined 
by  Rutherford  and  Soddy.  The  decay  curve,  like  the  decay 
curves  for  simple  radioactive  products,  is  exponential,  and 
UrX  loses  half  its  activity  in  about  22  days.  The  recovery 
curve  of  uranium  measured  by  the  (S  rays,  due  to  the  fresh 
production  of  UrX  in  the  uranium,  is  complementary  to  the 
decay  curve. 

From  analogy  with  the  corresponding  results  observed  in 
thorium  and  radium,  we  may  thus  conclude  that  uranium  pro- 
duces the  new  product  UrX  at  a  constant  rate.  Since  the  a 
ray  activity  is  unaffected  by  the  removal  of  UrX,  it  seems 
probable  that  the  uranium  atom  breaks  up  with  the  emission 
of  an  a  particle  and  then  becomes  the  atom  of  UrX.  This 
in  turn  breaks  up  with  the  expulsion  of  a  IB  particle.  The 
product  resulting  from  the  transformation  of  UrX  is  either 
inactive,  or  active  to  such  a  feeble  degree  that  its  transformation 
cannot  be  directly  followed  by  the  electric  method. 

The  changes  taking  place  in  uranium  are  diagram matically 
illustrated  below. 

1  Becquerel:  Comptes  rendus,  cxxxi,  p.  137  (1900) ;  cxxxiii,  p.  977  (1901). 


URANIUM  AND   ACTINIUM  163 

a  particle 

Uranium  atom  y  ray 

\ 

Atom  of  UrX >  (3  particle 

\ 

It  has  been  pointed  out  in  the  last  chapter  that  UrX  probably 
undergoes  one  or  more  further  changes  of  a  long  period,  possi- 
bly rayless  in  character,  and  is  finally  transformed  into  radium. 

There  are  several  points  of  interest  in  connection  with  the 
ft  ray  activity  exhibited  by  uranium.  Meyer  and  Schweidler l 
drew  attention  to  some  remarkable  variations  of  the  ft  activity 
of  uranium  during  crystallization  under  various  conditions. 
This  activity  varies  in  a  most  capricious  manner,  as  if  the 
process  of  crystallization  had  some  direct  effect  on  the  rate 
of  transformation  of  UrX.  Some  later  experiments  made  by 
Dr.  Godlewski 2  in  the  laboratory  of  the  writer  finally  led  to 
a  simple  explanation  of  these  puzzling  phenomena  observed  by 
Meyer  and  Schweidler. 

Some  uranium  nitrate  was  heated  and  sufficient  water  added 
for  complete  solution.  A  small  dish  containing  the  heated 
solution  was  then  placed  under  a  ft  ray  electroscope.  The  ft 
ray  activity  of  the  solution  remained  sensibly  constant  during 
the  cooling  of  the  solution,  but  the  moment  crystallization  com- 
menced at  the  bottom  of  the  dish,  the  ft  ray  activity  increased 
rapidly,  and  reached  several  times  its  initial  value  at  the  com- 
pletion of  the  crystallization.  After  reaching  a  maximum,  the 
activity  gradually  diminished  again,  and  about  a  week  later 
had  reached  a  value  equal  to  that  of  the  uranium  nitrate  before 
solution. 

Another  simple  experiment  was  then  made.  A  cake  of 
crystals  so  formed  was  removed  from  the  dish  immediately 
after  crystallization  was  completed,  and  inverted  under  the  elec- 
troscope. The  ft  ray  activity  was  much  less  than  for  the 
other  side  of  the  cake,  and  gradually  increased  again  to  the 

1  Meyer  and  Schweidler:  Wien  Ber.,  cxiii,  July,  1904. 

2  Godlewski:  Phil.  Mag.,  July,  1905.  ' 


164  RADIOACTIVE   TRANSFORMATIONS 

normal  value.  The  explanation  of  this  result  is  as  follows. 
UrX  is  more  soluble  in  water  than  uranium  itself.  When  the 
crystallization  starts  at  the  bottom  of  the  dish,  the  UrX  is 
pushed  towards  the  surface  of  the  solution.  The  $  rays  enter- 
ing the  electroscope  have  on  an  average  to  pass  through  a  less 
depth  of  uranium  than  before.  The  ft  ray  activity  will  thus 
increase  until  the  crystallization  is  complete.  The  lower  sur- 
face of  the  plate  of  crystals  will  contain  less  than  the  normal 
amount  of  UrX,  and  consequently  will  show  a  smaller  ft  ray 
effect.  The  gradual  decrease  of  the  /3  ray  activity  of  the 
upper  surface,  and  the  increase  of  activity  of  the  lower,  appears 
to  be  due  to  a  diffusion  of  the  UrX  through  the  mass  of 
crystals.  This  process  continues  until  the  UrX  is  again  uni- 
formly distributed  throughout  the  crystalline  mass.  This  dif- 
fusion takes  place  comparatively  rapidly  even  in  a  plate  of 
completely  dry  crystals.  An  effect  of  this  kind,  which  is  quite 
likely  to  occur  in  any  mixture  of  products  differing  in  solu- 
bility, shows  how  much  care  is  necessary  in  interpreting  varia- 
tions of  activity  in  a  mass  of  substance  which  has  just  been 
subjected  to  chemical  treatment. 

The  fact  that  UrX  is  more  soluble  in  water  than  uranium 
can  be  simply  utilized  to  effect  a  partial  separation  of  UrX. 
If  uranium  nitrate  is  dissolved  in  a  slight  excess  of  water,  the 
liquid  left  on  the  surface  after  crystallization  contains  a  large 
fraction  of  the  total  amount  of  the  UrX  originally  present  in 
the  uranium. 

CHANGES  IN  ACTINIUM 

Shortly  after  the  discovery  of  radium  and  polonium,  Debierne 
noted  the  presence  of  a  new  radioactive  substance  in  pitch- 
blende residues  which  he  called  actinium.  This  was  removed 
from  the  radioactive  mineral  with  the  thorium,  but  can  be 
separated  from  it  by  suitable  methods.  Very  little  was  known 
for  several  years  about  the  radioactive  peculiarities  of  this  sub- 
stance. In  the  meantime,  Giesel  had  independently  observed 
that  a  new  radioactive  substance  was  removed  with  the  lantha- 
num and  cerium  present  in  the  radioactive  mineral.  This 


URANIUM   AND   ACTINIUM  165 

substance  emitted  very  freely  a  short-lived  emanation,  and  it 
was  for  this  reason  that  he  first  termed  it  the  "emanating 
substance  "  —  a  name  which  was  later  changed  to  "emanium." 
Debierne  found  that  actinium  gave  out  an  emanation  which  lost 
half  its  activity  in  3.9  seconds.  Later  work  by  various  ob- 
servers has  shown  that  the  emanation  and  the  excited  activity 
produced  by  emanium  and  actinium  have  the  same  rates  of 
decay. 

The  active  constituent  present  in  the  actinium  of  Debierne 
is  thus  identical  with  that  in  the  "emanium  "  of  Giesel,  and  the 
original  name  "actinium  "  will  in  consequence  be  used  for  this 
substance.  Actinium  has  not  yet  been  separated  in  a  suffi- 
ciently pure  state  to  examine  its  atomic  weight  or  spectrum. 

Very  active  preparations  of  actinium  have  already  been  ob- 
tained by  Giesel  and  Debierne,  and  it  seems  probable  that  in 
the  pure  state  actinium  will  prove  to  be  of  the  same  order  of 
activity  as  radium.  The  emanation  is  given  out  very  freely 
from  the  preparations  of  Giesel,  and  excites  phosphorescence 
on  a  zinc  sulphide  screen  brought  near  it.  The  phenomenon 
of  scintillations  is  shown  by  actinium  rays  to  an  even  more 
marked  degree  than  by  the  a  rays  of  radium.  The  continuous 
and  rapid  emission  of  a  short-lived  emanation  from  actinium 
can  be  simply  illustrated  by  a  very  striking  experiment.  A 
small  quantity  of  actinium,  enclosed  in  a  paper  envelope,  is 
placed  on  a  zinc  sulphide  screen.  The  a  particles  emitted  from 
the  mass  of  the  substance  are  stopped  by  the  paper,  but  the 
emanation  readily  diffuses  through  it  into  the  surrounding  air. 
The  a  particles  expelled  from  the  emanation  produce  luminosity 
in  the  zinc  sulphide  screen.  On  examination  with  a  lens,  this 
light  is  seen  to  be  made  up  of  a  multitude  of  brilliant  scintilla- 
tions. A  puff  of  air  removes  the  emanation,  and  the  luminosity 
disappears  for  a  moment,  but  returns  almost  immediately  as  a 
fresh  amount  of  emanation  is  supplied.  The  luminosity  rapidly 
spreads  from  the  actinium  over  the  screen  by  the  process  of 
diffusion.  The  slightest  current  of  air  produces  a  marked  wav- 
ering effect  on  the  luminosity  and  displaces  the  luminosity  in 
the  direction  of  the  air  current. 


166  RADIOACTIVE   TRANSFORMATIONS 

Actinium  gives  out  a,  /3,  and  7  rays.  These  radiations  have 
been  examined  by  Godlewski.1  The  /3  rays  are  apparently 
fairly  homogeneous,  and  have  less  power  of  penetration  than  the 
corresponding  rays  from  other  active  substances.  This  shows 
that  the  /3  particles  are  all  projected  at  about  the  same  velocity, 
and  that  this  velocity  is  less  than  that  of  the  average  /3  rays 
from  other  substances. 

The  7  rays  also  have  much  less  penetrating  power  than  those 
from  radium.  It  seems  not  unlikely  that  the  absence  of  very 
penetrating  7  rays  is  connected  with  the  absence  of  swiftly 
moving  {3  particles,  for  it  is  probable  that  the  {3  particle,  which 
is  projected  from  radium  with  a  velocity  nearly  that  of  light, 
will  give  rise  to  a  more  penetrating  pulse  than  one  projected 
at  a  much  lower  speed. 

In  radioactive  properties,  actinium  shows  a  remarkable  simi- 
larity to  thorium.  It  emits  a  short-lived  emanation,  and  this 
is  transformed  into  an  active  deposit  which  is  concentrated  on 
the  negative  electrode  in  an  electric  field. 

The  activity  of  the  deposit  obtained  by  a  long  exposure  to 
the  emanation  subsequently  diminishes,  and  ten  minutes  after 
removal  from  the  emanation,  decays  exponentially  with  a  period 
of  about  34  minutes.  Miss  Brooks  2  showed  that  the  curves  of 
excited  activity  for  a  short  exposure  exhibited  the  same  general 
behavior  as  the  corresponding  curves  obtained  for  the  active 
deposit  of  thorium.  The  activity  at  first  increased,  passed 
through  a  maximum  after  about  8  minutes,  and  finally  decayed 
exponentially,  with  a  period  of  34  minutes. 

These  results  admit  of  the  same  explanation  as  in  the  case  of 
the  active  deposit  of  thorium.  The  emanation  which  gives  out 
a  rays  changes  into  a  rayless  product,  actinium  A,  which  is 
half  transformed  in  34  minutes.  This  changes  into  another 
substance  called  actinium  B,  which  is  half  transformed  in 
about  2  minutes,  and  emits  a,  /?,  and  7  rays. 

The  choice  of  the  2  minute  period  for  actinium  B  rather  than 
.for  actinium  A  followed  from  an  observation  of  Miss  Brooks. 

1  Godlewski:  Phil.  Mag.,  Sept.,  1905. 

2  Miss  Brooks:  Phil.  Mag.,  Sept.,  1904. 


URANIUM  AND  ACTINIUM  16T 

The  active  deposit,  obtained  on  a  platinum  plate,  was  dissolved 
in  hydrochloric  acid.  The  solution  was  then  electrolyzed,  and 
an  active  substance  which  emitted  a  rays  was  obtained  on  one 
of  the  electrodes.  This  lost  its  activity  exponentially  with 
the  period  of  about  1.5  minutes.  This  result  shows  that 
actinium  B,  which  emits  rays,  must  have  the  shorter  period. 

The  analogy  with  thorium  became  still  closer  when  God- 
lewski 1  and  Giesel  2  independently  separated  from  actinium  a 
very  active  substance  called  actinium  X.  This  was  effected 
by  precipitation  with  ammonia  in  exactly  the  same  way  as  is 
required  for  the  separation  of  ThX  from  thorium.  The  actin- 
ium X  after  precipitation  of  the  actinium  remains  behind  in 
the  filtrate  mixed  with  actinium  A  and  B.  Godlewski  found 
that  actinium  X  lost  its  activity  exponentially  with  a  period  of 
about  10  days.  The  actinium,  freed  from  actinium  X,  at  the 
same  time  recovered  its  activity.  There  are,  however,  several 
interesting  points  of  difference  in  the  chemical  separation  of 
actinium  X  and  ThX  from  their  respective  elements.  In  the 
case  of  thorium,  thorium  A  and  B  are  only  slightly  soluble  in 
ammonia,"  and  consequently  are  not  removed  with  the  ThX. 
Quite  the  reverse  holds  for  actinium.  The  active  deposit  is 
readily  soluble  in  ammonia,  and  consequently  is  separated  with 
actinium  X. 

After  removal  of  actinium  X  by  successive  precipitations,  the 
actinium  itself  retains  only  a  small  proportion  of  its  normal 
activity,  while  in  the  case  of  thorium,  the  residual  a  ray  activity 
is  about  one  quarter  of  the  total.  It  seems  probable  that,  if  the 
actinium  were  completely  freed  from  actinium  X  and  its  sub- 
sequent products,  the  element  itself  would  show  no  activity 
measured  by,  the  a  or  /3  rays,  or,  in  other  words,  that  actinium 
itself  is  a  rayless  product.  From  the  results  of  Hahn,  dis- 
cussed on  page  168,  it  has  already  been  pointed  out  that  thorium 
freed  from  radiothorium  may  also  prove  to  be  a  rayless  substance.3 

1  Godlewski:  Phil.  Mag.,  July,  1905. 

2  Giesel:  Jahrbuch.  d.  Radioaktivitat,  i,  p.  358  (1904). 

3  Hahn  (Nature,  April  12,  1906)  has  recently  separated  another  product  from 
actinium  which  he  has  called  "  radioactinium."    This  product  is  intermediate  between 
actinium  and  actinium  X,  emits  a  rays,  and  has  a  period  of  transformation  of  about 


168  RADIOACTIVE   TRANSFORMATIONS 

Godlewski  showed  that  the  emanation  from  actinium  was  a 
direct  product  of  actinium  X,  and  not  of  actinium  itself.  In 
this  respect,  ThX  and  actinium  X  have  very  similar  properties. 
The  transformations  taking  place  in  actinium  are  shown  in 
Fig.  40. 

On  comparison  of  the  changes  taking  place  in  actinium  and 
thorium  (see  Fig.  41)  the  similarity  in  the  succession  of  changes 
in  the  two  substances  is  very  noteworthy.  Not  only  are  the 
products  equal  in  number,  but  the  corresponding  products  are 
closely  allied  in  general  chemical  and  physical  properties.  The 
active  deposit  of  actinium  differs  somewhat  from  that  of  thorium 


ACT//V/VAJ.      ACTX-      EMANATION.     ACT  A. 

FIG.  40. 
Actinium  and  its  family  of  products. 

in  the  ease  with  which  it  is  dissolved  by  various  solutions  and 
the  lower  temperature  at  which  it  is  volatilized. 

This  similarity  in  the  radioactive  changes  of  the  two  sub- 
stances indicates  that  the  atoms  of  actinium  and  thorium,  while 
chemically  distinct,  are  very  similarly  constituted,  and  that 
when  once  the  process  of  disintegration  is  started,  the  atom  of 
both  substances  passes  through  a  similar  succession  of  changes. 

CONNECTION  BETWEEN  THE  RADIOELEMENTS 

The  series  of  transformations  taking  place  in  the  radioele- 
ments  are  shown  in  Fig.  41. l 

20  days.  Actinium  itself  is  a  rayless  product.  Godlewski  had  unknowingly  separated 
this  product  from  his  actinium,  for  otherwise  the  actinium  would  have  emitted  a  rays, 
due  to  the  presence  of  radioactinium.  Levin  has  found  that  actinium  X  does  not  emit 
£  rays.  The  £  rays  from  actinium  arise  only  from  the  product  actinium  B. 

1  In  the  diagram  (Fig.  41),  the  products  radiothorium,  radioactinium,  should  be 
introduced  between  thorium  and  thorium  X,  actinium  and  actinium  X,  respectively. 


URANIUM   AND   ACTINIUM  169 

The  substances  thorium,  radium,  and  actinium  exhibit  many 
interesting  points  of  similarity  in  the  course  of  their  transfor- 
mation. Each  gives  rise  to  an  emanation  whose  life  is  short 
compared  with  that  of  the  primary  element  itself.  Such  experi- 
ments as  have  yet  been  made,  indicate  that  these  emanations 
have  no  definite  combining  properties,  but  belong  apparently 


UF\ANIUM        UK-  X 


i         i         s  ".\ 

6-6-6-0-& 


THORIUM       TH.  X  e/*A/V.  TH.A  TH.B 


ACTTVC      "*"  O«  P06JT 


ACTINIUM    ACT.  X         EMAN-         ACT.  A         ACT.  S 


ACT/VC     DEPOSIT 


ACT/ve  DEPOSIT  f^AflO  CHAN«,E  ACT/Vfi  DEPOSIT  SLOW 

FIG.  41. 
The  radioelements  and  their  family  of  products. 

to  the  helium-argon  group  of  inert  gases.  In  each  case,  the 
emanation  gives  rise  to  a  non-volatile  substance  which  is  depos- 
ited on  the  surface  of  bodies  and  is  concentrated  on  the  nega- 
tive electrode  in  an  electric  field.  The  changes  in  these  active 
deposits  are  also  very  similar,  for  each  gives  rise  to  a  ray  less 
product,  followed  by  a  product  which  emits  all  three  types  of 
rays.  In  each  case,  also,  the  rayless  product  has  a  longer  period, 


170  RADIOACTIVE   TRANSFORMATIONS 

or,  in  other  words,  is  a  more   stable   substance  than  the  ray 
product  which  results  from  its  transformation. 

The  disintegration  of  the  corresponding  products  thorium  B, 
actinium  B,  and  radium  C  is  of  a  more  violent  character  than 
is  observed  in  the  other  products,  for  not  only  is  an  a  particle 
expelled  at  a  greater  speed,  but  a  /3  particle  is  also  thrown 
off  at  great  velocity.  After  this  violent  explosion  within  the 
atom,  the  resulting  atomic  system  sinks  into  a  more  permanent 
state  of  equilibrium,  for  the  succeeding  products  thorium  C  and 
actinium  C  have  not  so  far  been  detected  by  radioactive  methods, 
while  radium  D  is  transformed  at  a  very  slow  rate. 

This  similarity  in  the  properties  of  the  various  families  of 
products  is  too  marked  to  be  considered  a  mere  coincidence, 
and  indicates  that  there  is  some  underlying  law  which  governs 
the  successive  stages  of  the  disintegration  of  all  the  radioele- 
ments.  The  transformation  products  mark  the  distinct  stages 
in  the  career  of  disintegration  of  the  atoms,  and  represent  the 
halting  places  where  the  atoms  are  able  to  exist  for  an  appre- 
ciable time  before  again  breaking  up  into  other  more  or  less 
stable  configurations. 

The  interesting  question  arises  whether  the  atom  after  losing 
an  a  particle  is  able  to  exist  for  a  short  time  in  more  than  one 
stable  form.  After  the  expulsion  of  an  a  particle  with  explo- 
sive violence,  there  must  result  a  rearrangement  of  the  parts  of 
the  atom  to  form  a  permanently  or  temporarily  stable  system. 
It  is  conceivable  that  more  than  one  fairly  stable  arrangement 
may  be  possible,  and,  in  such  a  case,  two  or  more  products  of 
disintegration  must  be  produced  in  addition  to  the  expelled  a 
particles.  These  stable  atomic  systems,  although  of  equal 
atomic  weights,  would  exhibit  differences  in  chemical  properties, 
and  it  should  be  possible  to  separate  them  from  one  another.  It 
is  not  necessary  that  these  products  should  be  formed  in  equal 
amount.  One  might  exist  in  comparatively  large  amount 
compared  with  the  others. 

There  is  in  addition  another  possibility  to  be  borne  in  mind. 
The  violent  disturbance  in  the  atom  resulting  in  the  expulsion 
of  an  a  particle  may  cause  an  actual  breaking  up  of  the  main 


URANIUM  AND  ACTINIUM  171 

atom  into  two  parts,  and  thus  give  rise  to  an  equal  number  of 
atoms  of  different  atomic  weights  in  addition  to  the  a  particle. 
For  example,  such  an  effect  might  arise  during  the  violent 
disintegration  of  radium  C  or  thorium  B. 

So  far,  it  has  not  been  found  necessary  to  choose  between 
these  theories  to  explain  the  transformation  products  of  the 
different  elements.  The  disintegration  in  each  case  results  in 
the  appearance  of  only  one  substance  in  addition  to  the  expelled 
particles.  It  is  not  unlikely,  however,  that  a  still  closer  exami- 
nation of  the  radioelements  may  show  the  existence  of  prod- 
ucts which  lie  outside  the  main  line  of  descent.  The  method 
of  electrolysis  has  already  proved  of  great  value  in  separating 
products  of  the  transformation  of  radioelements  which  are  pres- 
ent in  infinitesimal  amount  in  a  solution,  and  its  possibilities 
in  this  direction  are  by  no  means  exhausted. 

RAYLESS  TRANSFORMATIONS 

We  have  seen  that  the  great  majority  of  the  products  break 
up  with  the  expulsion  of  an  a  particle;  in  addition,  a  small 
number  emit  a  ft  particle  with  its  accompaniment  the  7  rays, 
while  a  few  emit  only  a  ft  particle.  There  is  also  a  special 
class  of  product  which  does  not  emit  rays  at  all. 

It  has  been  shown  that  two  of  these  rayless  products  exist 
in  radium  and  actinium,  and  probably  two  in  thorium.  The 
method  of  showing  the  existence  of  such  rayless  products  and  of 
determining  their  physical  and  chemical  properties  has  already 
been  discussed  in  previous  chapters.  Since  a  rayless  product 
does  not  emit  any  ionizing  type  of  radiation,  its  presence  can 
only  be  observed  indirectly  by  examination  of  the  variation  in 
the  amount  of  the  succeeding  product.  By  such  methods,  we 
are  enabled  not  only  to  determine  the  period  of  change  of  the 
rayless  product,  but  also  its  more  marked  chemical  and  physical 
properties. 

These  products  are  apparently  similar  in  all  respects  to  the 
ray  products,  with  the  exception  that  there  is  no  evidence  of  the 
emission  of  a  or  ft  particles.  They  are  unstable  substances 
which  break  up  according  to  the  same  law  as  the  other  active 


172  RADIOACTIVE   TRANSFORMATIONS 

products,  and  give  rise  to  another  substance  of  different  phys- 
ical and  chemical  properties. 

There  are  two  general  ways  of  regarding  the  transformation 
of  a  rayless  product.  In  the  first  place,  it  may  be  supposed 
that  the  transformation  consists,  not  in  an  actual  expulsion  of 
a  part  of  the  atomic  system,  but  in  a  rearrangement  of  the 
component  parts  of  the  atom  to  form  a  new  temporarily  stable 
system.  On  such  a  view,  the  atom  of  the  rayless  product  has 
the  same  atomic  weight  as  the  succeeding  product,  but  differs 
from  it  so  materially  in  atomic  configuration  that  the  physical 
and  chemical  properties  are  quite  distinct.  The  two  products 
may  thus  be  considered  to  be  somewhat  analogous  to  the  case 
of  an  element  like  sulphur,  which  exists  in  two  distinct  forms. 
This  analogy  is,  however,  only  superficial,  for  the  atoms  of  the 
products  possess  entirely  different  chemical  and  physical  proper- 
ties whether  in  the  solid  state  or  in  solution. 

On  the  other  hypothesis,  the  transformation  of  a  rayless  prod- 
uct is  supposed  to  be  similar  in  character  to  that  of  a  ray 
product,  the  only  difference  being  that  the  a  particle  is  not 
expelled  with  sufficient  velocity  to  produce  appreciable  ioniza- 
tion  of  the  gas.  There  is  an  actual  loss  of  mass  during  the 
transformation,  but  this  loss  cannot  be  detected  by  the  electric 
method.  In  the  light  of  some  experimental  results,  discussed 
in  Chapter  X,  such  an  explanation  appears  not  improbable.  It 
is  there  shown  that  when  the  velocity  of  the  a  particle  falls 
below  about  40  per  cent  of  the  maximum  velocity  of  the  swift- 
est a  particle  from  radium,  viz.,  that  expelled  from  radium  C, 
the  photographic,  phosphorescent,  and  ionizing  properties  of  the 
a  particle  become  relatively  very  small.  Since  the  a  particle 
from  radium  C  is  projected  with  a  velocity  of  about  1/15  the 
velocity  of  light,  it  is  seen  that  an  a  particle  may  be  projected 
from  matter  at  a  great  speed,  and  yet  produce  a  comparatively 
weak  electrical  effect  compared  with  that  produced  by  a  particle 
projected  at  twice  its  velocity.  The  a  particle  from  radium  C 
produces  about  100,000  ions  in  the  gas  before  it  is  absorbed, 
and  consequently  the  electrical  effect  due  to  the  charged  a  par- 
ticles alone  would  be  insignificant  in  comparison  with  that  due 


URANIUM  AND   ACTINIUM  173 

to  the  ionization  of  the  gas  by  the  passage  of  the  swift  a  particle 
through  it.  Remembering  that  a  rayless  product  is  generally 
followed  by  a  product  which  emits  high  velocity  a  particles, 
the  strong  ionization  effect  due  to  the  latter  would  tend  to 
mask  completely  the  small  electrical  effect  due  to  the  rayless 
product  alone,  even  if  it  emitted  charged  a  particles  at  slow 
velocity. 

It  is  difficult  to  devise  experiments  to  decide  which  of  these 
two  hypotheses  as  to  the  nature  of  a  rayless  transformation  is 
correct,  but  the  view  that  there  is  an  expulsion  of  an  a  particle 
at  too  low  a  velocity  to  be  detected  by  ordinary  means  has  many 
points  in  its  favor. 

PROPERTIES  OF  THE  PRODUCTS 

We  have  seen  that,  with  a  few  exceptions,  the  products  of 
transformation  of  the  radioelements  exist  in  too  small  quanti- 
ties to  be  ever  detected  by  direct  measurement  of  their  weight 
or  volume.  Even  though  the  products  exist  in  infinitesimal 
amount  in  the  parent  matter,  the  property  of  emitting  ionizing 
radiations  serves  not  only  to  measure  their  rate  of  transforma- 
tion, but  also  to  deduce  some  of  their  physical  and  chemical 
properties. 

The  electric  method  has  been  utilized  as  an  accurate  means 
of  qualitative  and  quantitative  analysis  of  radioactive  matter 
which  is  present  in  extraordinarily  small  amount.  The  pres- 
ence of  10~n  gram  of  a  slowly  changing  substance  like  radium 
can  easily  be  observed,  while  in  the  case  of  more  rapidly  chang- 
ing matter  like  the  thorium  emanation,  one  hundred  millionth 
of  this  small  amount  is  readily  detectable.  In  fact,  as  has  been 
previously  pointed  out,  the  electric  method  is  easily  capable 
of  showing  the  presence  of  radioactive  matter  in  which  only 
one  atom  breaks  up  per  second,  provided  that  a  high  velocity 
particle  is  expelled  during  the  transformation. 

With  the  aid  of  the  electroscope,  the  range  of  possible  appli- 
cation of  chemical  methods  of  separation  has  been  enormously 
extended.  It  has  been  found  that  the  ordinary  methods  of 
•chemical  separation  of  substances,  whether  depending  on  dif- 


174  RADIOACTIVE   TRANSFORMATIONS 

ferences  in  solubility  or  volatility,  or  upon  electrolysis,  still 
apply  to  matter  existing  in  infinitesimal  proportion.  For  the 
detection  of  minute  amounts  of  active  matter,  the  electroscope 
far  transcends  in  delicacy  the  balance  or  even  the  spectroscope. 

The  study  of  radioactivity  has  thus  indirectly  furnished 
chemistry  with  new  methods  of  attack  on  the  properties  of 
active  matter  existing  in  extremely  small  quantity.  Much  still 
remains  to  be  done  in  this  new  field  of  work  whose  importance 
is  not  as  yet  sufficiently  recognized. 

Attention  has  already  been  drawn  to  the  radical  alteration  in 
properties  of  successive  transformation  products.  This  is  well 
exemplified  by  the  transformation  of  radium  into  its  emanation 
and  of  the  emanation  into  the  active  deposit.  Each  of  these 
substances  is  entirely  dissimilar  in  physical  and  chemical  nature 
to  the  others,  and,  but  for  other  evidence,  it  would  be  difficult 
to  believe  that  these  substances  were  derived  from  the  direct 
transformation  of  the  radium  atom. 

The  atom  at  most  stages  of  its  disintegration  loses  an  a 
particle,  which  has  an  apparent  mass  about  twice  that  of  the 
hydrogen  atom.  This  decrease  in  the  mass  of  the  atom  of 
about  one  per  cent  gives  rise  to  an  entirely  new  atomic  con- 
figuration whose  chemical  and  physical  properties  bear,  as  we 
have  seen,  no  obvious  relation  to  the  parent  atom.  This  radical 
change  in  the  properties  of  the  atom  is  not,  however,  very  sur- 
prising if  we  consider  chemical  analogies.  Elements  which  do 
not  differ  much  in  atomic  weight  often  possess  entirely  dis- 
similar properties,  and  thus  we  might  reasonably  expect  that  a 
decrease  of  the  atomic  mass  would  result  in  a  marked  change 
of  the  chemical  and  physical  nature  of  the  substance. 

There  cannot  now  be  any  doubt  that  the  radioactive  products 
arise  from  the  successive  transformations  of  the  atoms  of  matter, 
and  not  of  the  molecules.  Each  transformation  product  is  a 
distinct  element,  which  differs  only  from  the  well  known  in- 
active elements  in  the  comparative  instability  of  the  atoms  com- 
posing it.  There  can  be  no  doubt,  for  example,  that  the  radium 
emanation,  while  it  lasts,  is  a  new  elementary  substance  with  an 
atomic  weight  and  spectrum  that  distinguish  it  from  all  other 


URANIUM  AND  ACTINIUM  175 

elements.  If,  for  instance,  it  were  possible  to  examine  chemi- 
cally any  one  of  the  simple  products  in  a  time  which  is  short 
compared  with  its  period  of  transformation,  the  substance  would 
be  found  to  have  all  the  distinctive  properties  of  a  new  element. 
It  would  possess  a  definite  atomic  weight  and  spectrum  and  other 
distinctive  physical  and  chemical  properties.  In  regard  to  their 
position  as  elements,  no  line  of  demarcation  can  be  drawn 
between  the  comparatively  stable  elements  like  uranium,  tho- 
rium, and  radium,  and  their  rapidly  changing  products.  From 
a  radioactive  point  of  view,  the  atoms  of  these  substances  differ 
from  one  another  mainly  in  stability.  The  atoms  of  each  radio- 
element  may  differ  enormously  in  stability,  but  ultimately,  if 
sufficient  time  is  allowed,  all  of  these  substances  must  be  trans- 
formed through  a  succession  of  stages  and  disappear.  There 
will  finally  remain  only  the  inactive  or  stable  products  of  their 
decomposition. 

There  is  no  evidence  that  the  process  of  disintegration,  when 
once  started,  is  reversible  under  ordinary  conditions.  We  can 
obtain  the  radium  emanation  from  radium,  but  cannot  change 
the  emanation  back  again  into  radium.  The  question  whether 
this  process  has  been  reversible  under  some  possible  conditions 
existing  during  the  earth's  history  will  be  considered  later  in 
Chapter  IX. 

LIFE  OF  THE  RADIOELEMENTS 

We  have  seen  that  every  simple  product  which  emits  a 
radiation  decreases  in  amount  on  account  of  its  transformation 
into  another  substance.  The  rate  of  transformation  is  directly 
porportional  to  the  constant  A,  and  inversely  proportional  to  the 
period  of  transformation.  The  period  of  transformation  of  any 
simple  product  may  be  taken  as  a  comparative  measure  of  the 
stability  of  the  atoms  composing  it.  It  is  at  once  seen  that  the 
atomic  stability  of  the  products  whose  rate  of  transformation  has 
been  directly  measured  varies  over  an  enormous  range.  For  ex- 
ample, the  atoms  of  radium  F,  which  are  half  transformed  in 
140  days,  are  over  three  million  times  as  stable  as  the  atoms  of 
the  actinium  emanation  which  is  half  transformed  in  3. 9  seconds. 


176  RADIOACTIVE   TRANSFORMATIONS 

This  range  of  stability  of  the  atoms  is  still  further  extended 
when  we  include  the  atoms  of  the  primary  elements  uranium 
and  thorium. 

The  periods  of  transformation  of  these  substances  can  be 
approximately  deduced  by  comparison  of  their  a  ray  activities. 
Since  uranium  is  the  parent  of  radium,  the  relative  amounts  of 
uranium  and  radium  present  in  an  old  radioactive  mineral  are 
directly  proportional  to  the  periods  of  transformation  of  the  two 
substances.  Now  it  has  been  shown  that  3.8  x  10~7  gram  of 
radium  is  present  per  gram  of  uranium  in  any  radioactive 
mineral.  Since  radium  is  half  transformed  in  about  1800 

107 
years,  uranium  must  be  half  transformed  in  1300  x  o~o,  or  about 

3.4  x  109  years. 

The  period  of  transformation  of  thorium  is  probably  three 
or  four  times  greater  than  this,  since  its  activity  is  about  the 
same  as  that  of  uranium,  but  gives  rise  to  four  a  ray  products 
to  the  one  of  uranium.  In  order  that  a  large  fraction  of  any 
given  mass  of  uranium  may  be  transformed,  a  period  of  at  least 
ten  thousand  million  years  is  necessary. 

The  period  of  transformation  of  actinium  cannot  be  deter- 
mined until  it  has  been  obtained  in  a  pure  state.  If,  however, 
its  activity  is  of  the  same  order  as  that  of  radium,  its  period 
will  also  be  of  the  same  order. 

There  appears  to  be  no  obvious  relation  existing  between  the 
periods  of  the  successive  products  nor  between  the  periods  of 
the  different  families  of  products.  It  is  a  matter  of  remark, 
however,  that  a  substance  of  great  stability  is  generally  fol- 
lowed by  a  number  of  comparatively  unstable  products.  This  is 
well  exemplified  in  the  case  of  thorium,  radium,  and  actinium, 
where  most  of  their  known  products  suffer  rapid  transformation. 

CONNECTION  BETWEEN  URANIUM,  RADIUM,  AND  ACTINIUM 

The  connection  that  exists  between  uranium  and  radium,  and 
its  products,  radiolead  and  polonium,  has  been  clearly  brought 
out,  and  it  is  of  interest  to  examine  whether  any  similar  rela- 
tion exists  between  uranium  and  thorium,  and  uranium  and 


URANIUM  AND  ACTINIUM  177 

actinium.  The  latter  substance  is  always  found  in  uranium 
minerals,  and  since  it  probably  has  a  radioactive  life  comparable 
with  that  of  radium,  it  must  be  produced  in  some  way  from  the 
parent  substance  uranium. 

This  question  was  examined  by  Dr.  Boltwood  and  the  writer. 
If  actinium,  for  example,  was  a  product  of  uranium  in  the  main 
line  of  descent,  the  activity  due  to  actinium  or  to  a  uranium 
mineral  should  be  comparable  with  that  of  radium.  Since  there 
would  be  a  state  of  equilibrium  between  the  uranium  and 
actinium,  the  same  number  of  atoms  of  each  should  break  up 
per  second.  Since  actinium  has  four  a  ray  products  and 
radium  five,  the  activity  due  to  actinium  in  the  mineral  should 
be  comparable  with  that  due  to  radium  and  its  family  of 
products.  Experiment,  however,  showed  that  the  activity 
observed  in  Colorado  uraninite,  for  example,  was  almost  entirely 
due  to  the  uranium  and  radium  contained  in  it.  Its  activity 
due  to  actinium  was  certainly  only  a  small  portion  of  that  due 
to  radium  and  its  products.  It  seems  probable  that  actinium 
does  arise  from  uranium,  but  that  it  is  not  a  lineal  descendant 
of  uranium  in  the  same  sense  that  radium  is.  It  has  already 
been  pointed  out  that  in  some  of  the  transformations,  two 
distinct  transition  substances  may  be  produced.  It  appears 
likely  that  actinium  will  prove  to  be  derived  from  uranium  or 
one  of  its  products,  but  that  it  is  produced  in  much  less  amount 
than  the  other  product.  Such  a  relation  would  explain  the  con- 
nection that  apparently  exists  between  uranium  and  actinium, 
and  at  the  same  time  would  account  for  the  small  amount  of 
actinium  present. 

In  regard  to  the  connection  between  thorium  and  uranium, 
the  evidence  is  not  very  definite.  Many  minerals  contain 
uranium  and  very  little  thorium,  but  Strutt  has  shown  that 
every  thorium  mineral  examined  contains  some  uranium  and 
radium.  Strutt  has  suggested  that  thorium  is  the  parent  of 
uranium.  Such  a  relation  is  suggested  by  an  analysis  of  the 
mineral  thorianite.  This  mineral,  of  very  great  geological  age, 
is  found  in  Ceylon,  and  contains  about  70  per  cent  of  thorium 
and  12  per  cent  of  uranium.  The  uranium  in  this  mineral  may 

12 


178  RADIOACTIVE   TRANSFORMATIONS 

have  been  derived  from  the  decomposition  of  the  thorium. 
There  is,  however,  a  serious  objection  to  this  view,  for  the 
atomic  weight  of  thorium,  232.5,  is  less  than  that  of  the  usually 
accepted  atomic  weight  of  uranium,  238.5.  If  these  atomic 
weights  are  correct,  it  does  not  appear  likely  that  thorium  is  the 
parent  of  uranium,  unless  the  process  of  production  of  uranium 
from  thorium  is  very  different  from  that  usually  observed  in 
radioactive  transformations. 


CHAPTER  VIII 

THE  PRODUCTION  OF  HELIUM   FROM  RADIUM  AND   THE 
TRANSFORMATION   OF  MATTER 

THE  history  of  the  discovery  of  helium  possesses  some  fea- 
tures of  unusual  dramatic  interest.  In  1868,  Janssen  and 
Lockyer  observed  in  the  spectrum  of  the  sun's  chromosphere 
a  bright  yellow  line,  which  could  not  be  identified  with  that 
of  any  known  terrestrial  substance.  Lockyer  gave  the  name 
"  helium  "  to  this  supposed  new  element.  Further  comparison 
showed  that  certain  other  spectral  lines  in  the  chromosphere 
always  accompanied  the  yellow  line  and  were  probably  charac- 
teristic of  helium. 

The  spectrum  of  helium  is  observed  not  only  in  the  sun, 
but  also  in  many  of  the  stars ;  and  in  some  classes  of  stars,  now 
known  as  helium  stars,  the  spectrum  of  helium  predominates. 
No  evidence  of  the  existence  of  helium  on  the  earth  was  dis- 
covered until  1895.  Shortly  after  the  discovery  of  argon  in 
the  atmosphere,  by  Lord  Rayleigh  and  Sir  William  Ramsay,  a 
search  was  made  to  see  if  argon  could  be  obtained  from  mineral 
sources.  In  1895,  Miers  in  a  letter  to  "  Nature  "  drew  attention 
to  some  results  obtained  by  Hillebrande  of  the  U.  S.  Geological 
Survey  in  1891.  In  the  course  of  the  detailed  analysis  of  many 
of  the  minerals  containing  uranium,  a  considerable  quantity  of 
gas  was  found  by  Hillebrande 1  to  be  given  off  on  solution  of  the 
minerals.  At  the  time  he  thought  this  gas  was  nitrogen,  al- 
though attention  was  drawn  to  some  peculiarities  of  its  behav- 
ior as  compared  with  ordinary  nitrogen.  The  mineral  clevite 
especially  gave  off  a  large  quantity  of  gas  when  heated  or  dis- 
solved. Ramsay  procured  some  of  this  mineral  in  order  to  see 
whether  this  gas  might  prove  to  be  argon.  On  introducing  the 

i  Hillebrande,  Bull.  U.  S.  Geolog.  Survey,  No.  78,  p.  43  (1891). 


180  RADIOACTIVE   TRANSFORMATIONS 

gas  liberated  from  clevite  into  a  vacuum  tube,  a  spectrum  was 
observed  entirely  different  from  that  of  argon.1  The  spectrum 
was  carefully  examined  by  Lockyer 2  and  found  to  be  identical 
with  that  of  the  new  element  helium,  previously  discovered 
by  him  in  the  sun.  After  a  lapse  of  thirty  years  since  its  dis- 
covery in  the  sun,  helium  had  at  last  been  found  to  exist  in  the 
earth.  An  examination  of  the  properties  of  helium  soon  fol- 
lowed. It  has  a  well-marked  complex  spectrum  of  bright  lines, 
of  which  the  most  noticeable  is  a  bright  yellow  line  D3  close  to 
the  sodium  D  lines. 

It  is  a  light  gas  about  twice  as  dense  as  hydrogen,  and, 
excepting  the  latter,  has  a  lighter  atom  than  any  other  known 
element.  Like  argon,  it  refuses  to  combine  with  any  other  sub- 
stance, and  must  therefore  be  classed  with  the  group  of  chemi- 
cally inert  gases  discovered  by  Ramsay  in  the  atmosphere.  By 
measurement  of  the  velocity  of  sound  in  a  tube  filled  with 
helium,  the  ratio  of  its  two  specific  heats  was  found  to  be  1.66. 
The  ratio  for  diatomic  gases  like  hydrogen  and  oxygen  is  1.41. 
This  result  suggests  that  helium  is  monatomic,  i.  e.,  that  the 
helium  molecule  consists  of  only  one  atom ;  or,  in  other  words, 
that  the  atom  and  molecule  of  helium  are  the  same.  Since  the 
density  of  helium  was  found  to  be  1.98  times  that  of  hydrogen 
at  the  same  temperature  and  pressure,  and  since  the  hydrogen 
molecule  contains  two  atoms,  it  was  concluded  that  the  atomic 
weight  of  helium  is  twice  this  amount,  or  3.96.  It  must  be 
remembered  that  this  atomic  weight  has  been  determined  only 
from  density  observations,  since  helium  cannot  be  made  to  enter 
into  any  chemical  combination,  and  consequently  the  value 
given  for  its  atomic  weight  has  not  the  same  claim  to  accuracy 
as  the  atomic  weights  of  many  of  the  other  elements  which  have 
been  determined  by  more  rigorous  chemical  methods. 

Helium  was  found  to  exist  in  minute  proportion  in  the  atmos- 
phere. In  a  recent  paper  Ramsay  has  concluded  that  one  volume 
of  helium  is  present  in  245,000  volumes  of  air.  The  occurrence 
of  helium  in  certain  minerals  was  most  remarkable,  for  there  ap- 

1  Ramsay,  Proc.  Roy.  Soc.,  Iviii,  p.  65  (1895). 

2  Lockyer,  Proc.  Roy.  Soc.,  Iviii,  p.  67  (1895). 


THE   PRODUCTION   OF   HELIUM  181 

peared  no  obvious  reason  why  an  inert  gaseous  element  should 
be  found  associated  with  minerals,  which  in  many  cases  are 
impervious  to  the  passage  of  water  or  gases. 

Quite  a  new  light  was  thrown  on  this  subject  as  a  result  of 
the  discovery  of  radioactivity.  On  the  disintegration  theory  of 
radioactivity,  it  was  to  be  expected  that  the  final  or  inactive 
products  of  the  transformation  of  the  radioelemeiits  would  be 
found  in  the  radioactive  minerals.  Since  many  of  the  radioac- 
tive minerals  are  of  extreme  antiquity,  it  was  reasonable  to 
suppose  that  the  inactive  products  of  the  transformation  of 
radioactive  substances,  provided  they  did  not  escape,  would  be 
found  associated  in  some  quantity  with  the  radioactive  matter 
as  its  invariable  companions.  In  looking  for  a  possible  disin- 
tegration product,  the  occurrence  of  helium  in  all  radioactive 
minerals  was  noteworthy,  for  helium  is  mainly  found  in  minerals 
which  contain  a  large  quantity  of  uranium  or  of  thorium. 

For  these  and  other  reasons,  Rutherford  and  Soddy1  sug- 
gested that  helium  might  prove  to  be  a  disintegration  product 
of  the  radioelements.  Additional  weight  was  lent  to  this  sug- 
gestion by  the  writer's  discovery  that  the  a  particle  expelled  from 
radium  has  an  apparent  mass  about  twice  that  of  the  hydrogen 
atom  and  might  prove  to  be  an  atom  of  helium. 

In  the  beginning  of  1903,  thanks  to  Dr.  Giesel  of  Braun- 
schweig, small  quantities  of  pure  radium  bromide  were  placed 
on  the  market.  Ramsay  and  Soddy  obtained  30  milligrams  of 
the  bromide  and  proceeded  to  see  if  it  were  possible  to  detect 
the  presence  of  helium  in  the  gases  released  from  it.  In  the 
first  experiment  the  radium  bromide  was  dissolved  in  water  and 
the  accumulated  gases  drawn  off.  It  was  known  that  radium 
bromide  produced  hydrogen  and  oxygen,  and  these  gases  were 
removed  by  suitable  methods.  A  small  bubble  of  residual  gas 
was  obtained  which,  on  introduction  into  a  vacuum  tube,  showed 
the  characteristic  D3  line  of  helium.2  Using  another  and  some- 
what older  sample  of  radium  bromide,  lent  by  the  writer,  the 

1  Rutherford  and  Soddy:  Phil.  Mag.,  p.  582,  1902,  pp.  453  and  579,  1903. 

2  Ramsay  and  Soddy:  Nature,  July  16,  p.  246,  1903.    Proc.  Roy.  Soc.,  Ixxii, 
p.  204  (1903);  Ixxiii,  p.  346  (1904). 


182  RADIOACTIVE   TRANSFORMATIONS 

residual  gas  released  by  solution  of  the  radium  was  found  to 
give  a  complete  spectrum  of  helium. 

This  experiment  showed  that  helium  was  produced  by  radium 
and  retained  to  some  extent  in  the  solid  compound.  Further 
experiments  revealed  a  still  more  interesting  fact.  The  emana- 
tion from  the  60  milligrams  of  radium  bromide  was  condensed 
in  a  glass  tube  and  the  other  gases  pumped  out.  After  vola- 
tilization, the  emanation  was  introduced  into  a  small  vacuum 
tube.  The  spectrum  at  first  showed  no  sign  of  the  helium  lines, 
but  after  three  days  the  D3  line  of  helium  made  its  appearance, 
and  after  five  days  the  complete  spectrum  of  helium  was  ob- 
served. This  experiment  shows  that  helium  is  produced  from 
the  emanation,  for  no  evidence  of  its  presence  was  obtained 
immediately  after  the  introduction  of  the  emanation  into  the 
spectrum  tube. 

The  discovery  of  the  production  of  helium  by  the  radium 
emanation  was  of  great  importance,  as  it  showed  in  a  striking 
manner  the  extraordinary  nature  of  the  processes  occurring  in 
radium,  and  was  the  first  definite  evidence  of  the  possibility  of 
one  element  being  transformed  into  another  stable  element. 
The  experiments  were  not  easy  of  performance,  as  the  helium 
was  present  only  in  minute  amount,  and  the  experience  gained 
by  Ramsay  in  his  previous  work  on  the  rare  gases  in  the  atmos- 
phere was  of  the  greatest  practical  value  in  bringing  the  experi- 
ments to  such  a  successful  conclusion. 

The  production  of  helium  by  radium  has  been  confirmed  by 
a  number  of  experimenters.  Curie  and  Dewar1  made  in  this 
connection  a  most  interesting  experiment,  which  showed  con- 
clusively that  the  helium  was  produced  from  radium  and  could 
not  be  ascribed  to  a  possible  occlusion  of  helium  in  the  ra- 
dium bromide.  A  large  quantity  of  radium  chloride  was  in- 
troduced into  a  quartz  tube  and  the  radium  heated  to  fusion. 
The  emanation  and  gases  in  the  tube  were  pumped  out  and 
the  tube  sealed.  One  month  later,  Deslandres  examined  the 
spectrum  of  the  gases  in  the  tube  by  placing  layers  of  foil 
over  the  ends.  A  complete  spectrum  of  helium  was  observed, 

l  Curie  and  Dewar:  Comptes  rendus,  cxxxviii,  p.  190  (1904). 


THE  PRODUCTION   OF   HELIUM  183 

showing  that  the  helium  had  been  produced  from  the  radium 
in  the  interval. 

Recently  Debierne  l  has  found  that  helium  is  also  produced 
from  active  preparations  of  actinium.  This  result  shows  that 
the  helium  must  be  a  common  product  of  these  two  substances, 
which,  from  their  radioactive  and  chemical  behavior,  must  be 
regarded  as  distinct  elements. 

THE  POSITION  OF  HELIUM  AS  A  TRANSFORMATION 
PRODUCT  OF  RADIUM 

We  have  already  seen  that  radium  is  transformed  through  a 
long  succession  of  products,  each  of  which  has  some  distinctive 
physical  and  chemical  properties  and  a  definite  period  of  trans- 
formation. These  products  differ  from  the  ordinary  chemical 
elements  only  in  the  instability  of  their  component  atoms. 
They  must  be  regarded  as  transition  elements  with  a  limited  life, 
which  break  up  into  new  forms  of  matter  at  a  rate  independent 
of  our  control. 

The  distinction,  however,  between  helium  as  a  product  of 
radium  and  the  family  of  transition  products  is  mainly  one  of 
atomic  stability.  As  far  as  we  know,  helium  is  a  stable  element 
which  does  not  disappear,  but  in  the  case  of  all  the  radioactive 
products,  including  the  primary  sources  uranium  and  thorium, 
the  atoms  are  undoubtedly  unstable. 

It  is  now  necessary  to  consider  the  position  of  helium  as  a 
transformation  product  of  radium.  Some  have  considered  that 
helium  is  the  end  or  final  product  of  the  disintegration  of  the 
radium  atom,  but  for  this  there  is  no  experimental  evidence. 
We  have  seen  that  after  the  first  rapid  changes  in  the  active 
deposit  of  the  emanation,  there  is  produced  a  very  slow  transition 
substance,  radium  D.  If  helium  were  the  final  product  of  the 
transformation  of  the  radium  atom,  the  amount  produced  from 
the  emanation  in  the  course  of  a  few  days  would  be  infinitesi- 
mally  small.  In  addition  there  can  be  little  doubt  that  the  final 
active  product  of  radium,  viz.  radium  F  (polonium),  is  an  ele- 
ment of  high  atomic  weight. 

1  Debierne:  Comptes  rendus,  cxli,  p.  383  (1905). 


184  RADIOACTIVE   TRANSFORMATIONS 

The  evidence,  on  the  other  hand,  points  strongly  to  the  con- 
clusion that  the  helium  is  formed  by  the  a  particles  continuously 
shot  out  from  radium  and  its  products.  We  shall  see  later 
(Chapter  X)  that  the  experimental  evidence  shows  that  the  a 
particle  shot  out  from  the  different  a  ray  products  of  radium 
has  in  each  case  the  same  mass,  but  varies  in  velocity  for  the 
different  products. 

From  observation  of  the  deflection  of  the  rays  both  in  a  strong 
magnetic  and  a  strong  electric  field,  the  velocity,  and  the  value 
e/m  — the  ratio  of  the  charge  of  the  a  particle  to  its  mass  — 
have  been  accurately  measured.  The  ratio  e/m  is  very  nearly 
5  x  103.  Now  the  ratio  e/m  for  the  hydrogen  atom  liberated  in 
the  electrolysis  of  water  is  known  to  be  104.  If  we  assume  that 
the  a  particle  carries  the  same  charge  as  the  hydrogen  atom,  the 
mass  of  the  a  particle  is  twice  that  of  the  hydrogen  atom.  We 
are  here  unfortunately  confronted  with  several  possibilities  be- 
tween which  it  is  at  present  difficult  to  make  a  definite  choice. 

The  value  of  e/m  for  the  a  particle  may  equally  well  be 
explained  on  the  assumptions  that  the  a  particle  is  (1)  a  mole- 
cule of  hydrogen,  (2)  a  helium  molecule  carrying  twice  the 
charge  of  the  hydrogen  atom,  or  (3)  one  half  of  the  helium 
molecule  carrying  the  usual  ionic  charge. 

The  hypothesis  that  the  a  particle  is  a  molecule  of  hydrogen 
seems  for  many  reasons  improbable.  If  hydrogen  is  a  constitu- 
ent of  the  atoms  of  radioactive  matter,  it  is  to  be  expected  that 
it  would  be  expelled  in  the  atomic  and  not  in  the  molecular 
state.  In  all  cases  so  far  examined,  when  hydrogen  is  the  car- 
rier of  an  electric  charge,  the  value  of  e/m  is  104.  This  is  the 
value  to  be  expected  for  the  hydrogen  atom.  For  example, 
Wien  found  that  the  maximum  value  of  ejm  for  the  canal  rays 
or  positive  ions,  which  are  produced  in  an  exhausted  vacuum 
tube,  was  104.  In  addition,  it  seems  improbable  that,  even  if 
the  hydrogen  were  projected  initially  in  the  molecular  state,  it 
would  escape  decomposition  into  its  component  atoms  in  passing 
through  matter. 

When  it  is  remembered  that  an  a  particle  is  projected 
with  a  velocity  of  about  12,000  miles  per  second,  and  collides 


THE   PRODUCTION   OF   HELIUM  185 

with  every  molecule  in  its  path,  the  disturbance  set  up  in  the 
molecule  by  the  collisions  must  be  very  intense,  and  must  tend 
to  rupture  the  bonds  that  hold  the  atoms  of  the  molecule 
together.  Indeed,  it  seems  very  unlikely  that  the  hydrogen  mole- 
cule under  such  conditions  could  escape  decomposition  into  its 
component  atoms.  If  the  a  particle  were  a  hydrogen  molecule, 
a  considerable  amount  of  free  hydrogen  should  be  present  in 
old  radioactive  minerals  which  are  sufficiently  dense  to  prevent 
its  escape.  This  does  not  appear  to  be  the  case,  although  in 
some  minerals  there  is  a  considerable  quantity  of  water.  On 
the  other  hand,  the  comparatively  large  amount  of  helium  pres- 
ent supports  the  view  that  the  a  particle  is  connected  with 
helium.  A  strong  argument  in  favor  of  the  view  of  a  connection, 
between  helium  and  the  a  particle  rests  on  the  observed  facts  that 
helium  is  produced  by  actinium  as  well  as  by  radium.  The 
only  point  of  similarity  between  these  two  substances  lies  in  the 
expulsion  of  a  particles.  The  production  of  helium  by  both 
substances  is  at  once  obvious  if  the  helium  is  derived  from  the 
accumulated  a  particles,  but  is  difficult  to  explain  on  any  other 
hypothesis.  We  are  thus  reduced  to  the  view  that  either  the  a 
particle  is  a  helium  atoni  carrying  twice  the  ionic  charge,  or 
that  it  is  half  of  a  helium  atom  carrying  an  ionic  charge. 

The  latter  assumption  involves  the  conception  that  helium, 
while  behaving  as  a  chemical  atom  under  ordinary  chemical  and 
physical  conditions,  may  exist  in  a  still  more  elementary  state  as 
a  component  of  the  atoms  of  radioactive  matter,  and  that,  after 
expulsion,  the  a  particles  lose  their  charge  and  recombine  to 
form  atoms  of  helium. 

While  such  a  view  cannot  be  dismissed  as  inherently  improb- 
able, there  is  no  direct  evidence  in  its  favor.  On  the  other 
hand,  the  second  hypothesis  has  the  merit  of  greater  simplicity 
and  probability. 

On  this  view,  the  a  particle  is  in  reality  an  atom  of  helium 
which  is  either  expelled  with  a  double  ionic  charge  or  acquires 
this  charge  in  its  passage  through  matter.  Even  if  the  a  particle 
were  initially  projected  without  a  charge,  it  would  almost  certainly 
acquire  one  after  the  first  few  collisions  with  the  molecules  in  its 


186  RADIOACTIVE   TRANSFORMATIONS 

path.  We  know  that  the  a  particle  is  a  very  efficient  ionizer,  and 
there  is  every  reason  to  suppose  that  it  would  itself  be  ionized  by 
its  collision  with  the  molecules  in  its  path,  i.  £.,  it  would  lose  an 
electron,  and  would  consequently  itself  retain  a  positive  charge. 

If  the  a  particle  can  remain  stable  with  the  loss  of  two  elec- 
trons, these  electrons  would  almost  certainly  be  ejected  as  a 
result  of  the  intense  disturbance  arising  from  the  collision  of 
the  a  particle  with  the  molecules  in  its  path.  The  a  particle 
would  then  have  twice  the  ordinary  ionic  charge,  and  the  value 
of  e/m,  as  found  by  measurement,  would  be  quite  consistent 
with  the  view  that  the  a  particle  is  an  atom  of  helium. 

If  this  be  the  case,  the  actual  number  of  a  particles  expelled 
from  radium  would  be  only  one  half  of  that  deduced  on  the  as- 
sumption that  the  a  particle  carries  a  single  charge.  This  would 
make  the  rate  of  disintegration  of  radium  only  half  of  that  cal- 
culated in  Chapter  VI,  and  would  consequently  double  its  life. 

In  a  similar  way  this  assumption  would  reduce  the  calculated 
volume  of  the  emanation  released  from  one  gram  of  radium  from 
.8  c.  mms.  to  .4  c.  mms.  This  is  smaller  than  the  experimental 
value  —  about  1  c.  mm.  —  determined  by  Ramsay  and  Soddy, 
but  is  of  the  right  order  of  magnitude. 

On  the  above  assumptions,  the  volume  of  helium  produced  per 
year  per  gram  of  radium  can  readily  be  calculated.  If  each  a 
particle  carries  twice  the  ionic  charge,  experiment  shows  that 
1.25  X  1011  a  particles  are  expelled  per  second  from  one  gram 
of  radium  in  equilibrium.  The  number  expelled  per  year  is 
4.0  x  1018.  Since  one  cubic  centimetre  of  a  gas  at  standard  press- 
ure and  temperature  contains  3.6  x  1019  molecules,  the  volume 
of  helium  produced  per  year  per  gram  of  radium  is  .11  c.  cms. 

Ramsay  and  Soddy  made  an  estimate  of  the  rate  of  produc- 
tion of  helium  from  radium  in  the  following  manner.  The 
helium  produced  from  50  mgrs.  of  radium  bromide  kept  in  a 
closed  vessel  for  60  days  was  introduced  into  a  vacuum  tube. 
Another  similar  tube  was  placed  in  series  with  it  and  the 
amount  of  helium  in  the  latter  was  adjusted  until  the  discharge, 
passed  in  series  through  the  two  tubes,  showed  the  helium 
lines  with  about  the  same  intensity.  In  this  way,  they  deduced 


THE   PRODUCTION   OF  HELIUM  187 

the  volume  obtained  from  the  radium  to  be  0.1  cubic  mm. 
This  corresponds  to  a  rate  of  production  of  helium  per  gram  of 
radium  per  year  of  about  20  cubic  mms.  This  is  only  about  one 
fifth  of  the  theoretical  amount  calculated  above.  Ramsay  and 
Soddy  do  not  lay  much  stress  on  the  accuracy  of  their  estimate, 
as  they  consider  that  the  presence  of  a  trace  of  argon  may  have 
seriously  interfered  with  the  correctness  of  the  estimate  by  the 
spectroscopic  method.  An  accurate  measurement  of  the  rate  of 
production  of  helium  by  radium  would  be  of  the  utmost  value 
at  the  present  time  in  settling  the  connection  between  the  a 
particle  and  helium. 

If  the  a  particle  is  a  helium  atom,  the  greater  proportion  of 
the  a  particles  expelled  from  the  emanation  enclosed  in  a  small 
tube  will  be  projected  into  the  glass  envelope.  The  swiftest 
moving  particles,  viz.,  those  expelled  from  radium  C,  would 
probably  penetrate  the  glass  to  a  depth  of  about  1/20  of  a 
millimetre,  while  the  slower  moving  particles  would  be  stopped 
after  traversing  a  somewhat  shorter  distance. 

It  has  already  been  pointed  out  (page  88)  that  this  may 
explain  why  the  volume  of  the  emanation  in  the  first  ex- 
periment by  Ramsay  and  Soddy  shrank  almost  to  zero.  The 
helium  in  this  case  was  retained  in  the  glass.  In  the  second 
experiment  the  helium  may  have  diffused  from  the  glass  tube 
into  the  gas  again.  Ramsay  and  Soddy  endeavored  to  settle 
this  point  by  testing  whether  helium  was  released  by  heating  a 
glass  tube  in  which  the  emanation  had  been  enclosed  for  several 
days  and  then  removed.  The  spectroscope  momentarily  showed 
some  of  the  helium  lines,  but  these  were  soon  obscured  by  the 
presence  of  other  gases  liberated  by  the  heating  of  the  tube. 

AGE  OF  RADIOACTIVE  MINERALS 

The  helium  observed  in  the  radioactive  minerals  is  almost 
certainly  due  to  its  production  from  the  radium  and  other 
radioactive  substances  contained  therein.  If  the  rate  of  pro- 
duction of  helium  from  known  weights  of  the  different  radio- 
elements  were  experimentally  known,  it  should  thus  be  possible 
to  determine  the  interval  required  for  the  production  of  the 


188  RADIOACTIVE   TRANSFORMATIONS 

amount  of  helium  observed  in  radioactive  minerals,  or,  in  other 
words,  to  determine  the  age  of  the  mineral.  This  deduction  is 
based  on  the  assumption  that  some  of  the  denser  and  more  com- 
pact of  the  radioactive  minerals  are  able  to  retain  indefinitely  a 
large  proportion  of  the  helium  imprisoned  in  their  mass.  In 
many  cases  the  minerals  are  not  compact  but  porous,  and  under 
such  conditions  most  of  the  helium  will  escape  from  its  mass. 
Even  supposing  that  some  of  the  helium  has  been  lost  from 
the  denser  minerals,  we  should  be  able  to  fix  with  some  cer- 
tainty a  minimum  limit  for  the  age  of  the  mineral. 

In  the  absence  of  definite  experimental  data  on  the  rates  of  pro- 
duction of  helium  by  the  different  radioelements,  the  deductions 
are  of  necessity  somewhat  uncertain,  but  will  nevertheless  serve 
to  fix  the  probable  order  of  the  ages  of  the  radioactive  minerals. 

It  has  already  been  pointed  out  that  all  the  a  particles  ex- 
pelled from  radium  have  the  same  mass.  In  addition  it  has 
been  experimentally  found  that  the  a  particle  from  thorium  B 
has  the  same  mass  as  the  a  particle  from  radium.  This  would 
suggest  that  the  a  particles  projected  from  all  radioactive  sub- 
stances have  the  same  mass,  and  thus  consist  of  the  same  kind 
of  matter.  If  the  a  particle  is  a  helium  atom,  the  amount  of 
helium  produced  per  year  by  a  known  quantity  of  radioactive 
matter  can  readily  be  deduced  on  these  assumptions. 

The  number  of  products  which  expel  a  particles  are  now  well 
known  for  radium,  thorium,  and  actinium.  Including  radium  F, 
radium  has  five  a  ray  products,  thorium  five,  and  actinium  four. 
With  regard  to  uranium  itself,  there  is  not  the  same  certainty, 
for  only  one  product,  UrX,  which  emits  only  /3  rays,  has  so  far 
been  chemically  isolated  from  uranium.  The  a  particles  appar- 
ently are  emitted  by  the  element  uranium  itself ;  at  the  same 
time,  there  is  some  indirect  evidence  in  support  of  the  view  that 
uranium  contains  three  a  ray  products.  For  the  purpose  of 
calculation,  we  shall,  however,  assume  that  in  uranium  and 
radium  in  equilibrium,  one  a  particle  is  expelled  from  the 
uranium  for  five  from  the  radium. 

Let  us  now  consider  an  old  uranium  mineral  which  contains 
one  gram  of  uranium,  and  which  has  not  allowed  any  of  the 


THE   PRODUCTION  OF  HELIUM  189 

products  of  its  decomposition  to  escape.  The  uranium  and 
radium  are  in  radioactive  equilibrium  and  3.8  x  10~7  grams  of 
radium  are  present.  For  one  a  particle  emitted  by  the  ura- 
nium, five  are  emitted  by  the  radium  and  its  products,  including 
radium  F.  Now  we  have  shown  that  radium  with  its  four  a 
ray  products  probably  produces  .11  c.c.  of  helium  per  gram  per 
year.  The  rate  of  production  of  helium  by  the  uranium  and  ra- 
dium in  the  mineral  will  consequently  be  J  x  .11  x  3.8  X  10  ~7  — 
5.2  x  10~8  c.c.  per  year  per  gram  of  uranium. 

Now,  as  an  example  of  the  method  of  calculation,  let  us  con- 
sider the  mineral  fergusonite  which  was  found  by  Ramsay  and 
Travers  to  evolve  1.81  c.c.  of  helium  per  gram.  The  fergusonite 
contains  about  7  per  cent  of  uranium.  The  amount  of  helium 
contained  in  the  mineral  per  gram  of  uranium  is  consequently 
26  c.c. 

Since  the  rate  of  production  of  helium  per  gram  of  uranium 
and  its  radium  products  is  5.2  x  10~8  c.c.  per  year,  the  age  of 
the  mineral  must  be  at  least  26 -f- £.2  x  10~8J  years  or  500 
million  years.  This,  as  we  have  pointed  out,  is  a  minimum 
estimate,  for  some  of  the  helium  has  probably  escaped. 

We  have  assumed  in  this  calculation  that  the  amount  of 
uranium  and  radium  present  in  the  mineral  remains  sensibly 
constant  over  this  interval.  This  is  approximately  the  case, 
for  the  parent  element  uranium  probably  requires  about  1000 
million  years  to  be  half  transformed. 

As  another  example,  let  us  take  a  uranium  mineral  obtained 
from  Glastonbury,  Connecticut,  which  was  analyzed  by  Hille- 
brande.  This  mineral  was  very  compact  and  of  high  density, 
9.62.  It  contained  76  per  cent  of  uranium  and  2.41  per  cent  of 
nitrogen.  This  nitrogen  was  almost  certainly  helium,  and 
dividing  by  seven  to  reduce  to  helium  this  gives  the  percentage 
of  helium  as  0.344.  This  corresponds  to  19  c.c.  of  helium  per 
gram  of  the  mineral,  or  25  c.c.  per  gram  of  uranium  in  the  min- 
eral. Using  the  same  data  as  before,  the  age  of  the  mineral 
must  be  certainly  not  less  than  500  million  years.  Some  of  the 
uranium  and  thorium  minerals  do  not  contain  much  helium. 
Some  are  porous,  and  must  allow  the  helium  to  escape  readily. 


190  RADIOACTIVE   TRANSFORMATIONS 

A  considerable  quantity  of  helium  is,  however,  nearly  always 
found  in  the  compact  primary  radioactive  minerals,  which  from 
geologic  data  are  undoubtedly  of  great  antiquity. 

Hillebrande  made  a  very  extensive  analysis  of  a  number  of 
samples  of  minerals  from  Norway,  North  Carolina,  and  Connec- 
ticut, which  were  mostly  compact  primary  minerals,  and  noted 
that  a  striking  relation  existed  between  the  proportion  of 
uranium  and  of  nitrogen  (helium)  that  they  contained.  This 
relation  is  referred  to  in  the  following  words :  — 

"  Throughout  the  whole  list  of  analyses  in  which  nitrogen 
(helium)  has  been  estimated,  the  most  striking  feature  is  the 
apparent  relation  between  it  and  the  UO2.  This  is  especially 
marked  in  the  table  of  Norwegian  uraninites,  recalculated  from 
which  the  rule  might  almost  be  formulated  that,  given  either 
nitrogen  or  UO2,  the  other  can  be  found  by  simple  calculation. 
The  same  ratio  is  not  found  in  the  Connecticut  varieties,  but 
if  the  determination  of  nitrogen  in  the  Branchville  mineral  is  to 
be  depended  on,  the  rule  still  holds  that  the  higher  the  UO2  the 
higher  likewise  is  the  nitrogen.  The  Colorado  and  North  Caro- 
lina minerals  are  exceptions,  but  it  should  be  borne  in  mind  that 
the  former  is  amorphous,  like  the  Bohemian,  and  possesses  the 
further  similarity  of  containing  no  thoria,  although  zirconia 
may  take  its  place,  and  the  North  Carolina  mineral  is  so  much 
altered  that  its  original  condition  is  unknown." 

Very  little  helium,  however,  is  found  in  the  secondary  radio- 
active minerals,  i.  e.,  minerals  which  have  been  formed  as  a  result 
of  the  decomposition  of  the  primary  minerals.  These  minerals, 
as  Boltwood  has  pointed  out,  are  undoubtedly  in  many  cases  of 
far  more  recent  formation  than  the  primary  minerals,  and  conse- 
quently it  is  not  to  be  expected  that  they  should  contain  as 
much  helium.  One  of  the  most  interesting  deposits  of  a  second- 
ary uraninite  is  found  at  Joachimsthal  in  Bohemia,  from  which 
most  of  our  present  supply  of  radium  has  been  obtained.  This 
is  rich  in  uranium,  but  contains  very  little  helium. 

When  the  data  required  for  these  calculations  are  known  with 
more  definiteness,  the  presence  of  helium  in  radioactive  minerals 
will  in  special  cases  prove  a  most  valuable  method  of  computing 


THE   PRODUCTION   OF  HELIUM 


191 


their  probable  age,  and  indirectly  the  probable  age  of  the  geo- 
logical deposits  in  which  the  minerals  are  found.  Indeed,  it  ap- 
pears probable  that  it  will  prove  one  of  the  most  reliable  methods 
of  determining  the  age  of  the  various  geological  formations. 

SIGNIFICANCE  OF  THE  PRESENCE  OF  LEAD  IN 
RADIOACTIVE  MINERALS 

If  the  a  particle  is  a  helium  atom,  the  atomic  weights  of  the  suc- 
cessive a  ray  products  of  radium  must  differ  by  equal  steps  of  four 
units.  Now  we  have  seen  that  uranium  itself  probably  contains 
three  a  ray  products.  Since  the  atomic  weight  of  uranium  is 
238.5,  the  atomic  weight  of  the  residue  of  the  uranium  after  the 
expulsion  of  three  a  particles  would  be  238.5  —  12,  =  226.5. 
This  is  very  close  to  the  atomic  weight  of  radium  225,  which  we 
have  seen  is  produced  from  uranium.  Now  radium  emits  five  a 
ray  products  altogether,  and  the  atomic  weight  of  the  end  prod- 
uct of  radium  should  be  238.5  —  32,  =  206.5.  This  is  very  close 
to  the  atomic  weight  of  lead,  206.9.  This  calculation  suggests 
that  lead  may  prove  to  be  the  final  product  of  the  decomposition 
of  radium,  and  this  suggestion  is  strongly  supported  by  the 
observed  fact  that  lead  is  always  found  associated  with  the 
radioactive  minerals,  and  especially  in  those  primary  minerals 
which  are  rich  in  uranium. 

The  possible  significance  of  the  presence  of  lead  in  radioactive 
minerals  was  first  noted  by  Boltwood,1  who  has  collected  a  large 
amount  of  data  bearing  on  this  question. 

The  following  table  shows  the  collected  results  of  an  analysis 
of  different  primary  minerals  made  by  Hillebrande :  — 


Locality. 

Percentage  of 
uranium. 

Percentage  of 
lead. 

Percentage  of 
nitrogen. 

Glastonbury,  Connecticut 
Branchville,   Connecticut 
North  Carolina    .... 
Norway        .     .              . 

70-72 

74-75 
77 
56-66 

3.07-3.26 
4.35 
4.20-4.53 
7  62-13  87 

2.41 

2.63 

1.03-1.28 

Canada                                . 

65 

1049 

086 

Boltwood:  Phil.  Mag.,  April,  1905 ;  Amer.  Journ.  Science,  Oct.,  1905. 


192  RADIOACTIVE   TRANSFORMATIONS 

Five  samples  were  taken  of  the  minerals  from  Glastonbury, 
three  from  Branch  ville,  two  from  North  Carolina,  seven  from 
Norway,  and  one  from  Canada.  In  minerals  obtained  from  the 
same  locality,  there  is  a  comparatively  close  agreement  between 
the  amounts  of  lead  contained  in  them.  If  helium  and  lead  are 
both  products  of  the  decomposition  of  the  uranium  radium  miner- 
als, there  should  exist  a  constant  ratio  between  the  percentage 
of  lead  and  helium  in  the  minerals.  The  percentage  of  helium  is 
obtained  from  the  above  table  by  dividing  the  nitrogen  percentage 
by  seven.  Since  probably  .eight  a  particles  are  emitted  from  the 
decomposition  of  uranium  and  radium  for  the  production  of  one 

8x4 
atom  of  lead,  the  weight  of  helium  formed  should  be  ^7^-^  =.165 


of  the  weight  of  lead.  This  is  based  on  the  assumption  that  all 
the  helium  formed  is  imprisoned  in  the  minerals.  The  ratio 
actually  found  is  about  .11  for  the  Glastonbury  minerals,  .09  for 
the  Branch  ville  minerals,  and  about  .016  for  the  Norway  min- 
erals. It  will  be  noted  that  in  all  cases  the  ratio  of  helium  to 
lead  is  less  than  the  theoretical  ratio,  indicating  that  in  some 
cases  a  large  proportion  of  the  helium  formed  in  the  mineral  has 
escaped.  In  the  case  of  the  Glastonbury  minerals,  the  observed 
ratio  is  in  good  agreement  with  theory. 

If  the  production  of  lead  from  radium  is  well  established,  the 
percentage  of  lead  in  radioactive  minerals  should  be  a  far  more 
accurate  method  of  deducing  the  age  of  the  mineral  than  the 
calculation  based  on  the  volume  of  helium,  for  the  lead  formed 
in  a  compact  mineral  has  no  possibility  of  escape. 

While  the  above  considerations  are  of  necessity  somewhat 
conjectural  in  the  present  state  of  our  knowledge,  they  are  of 
value  as  indicating  the  possible  methods  of  attacking  the  ques- 
tion as  to  the  final  products  of  the  decomposition  of  the  radio- 
active minerals.  From  a  study  of  the  data  of  analyses  of 
radioactive  minerals,  Boltwood  has  suggested  that  argon,  hy- 
drogen, bismuth,  and  some  of  the  rare  earths  possibly  owe 
their  origin  to  the  transformation  of  the  primary  radioactive 
substances. 

It  does  not  appear  likely  that  we  shall  be  able  for  many  years 


THE   PRODUCTION   OF   HELIUM  193 

to  prove  or  disprove  experimentally  that  lead  is  the  final  prod- 
uct of  radium.  In  the  first  place,  it  is  difficult  for  the  experi- 
menter to  obtain  sufficient  radium  for  working  material,  and,  in 
the  second  place,  the  presence  of  the  slowly  transformed  product 
radium  D  makes  a  long  interval  necessary  before  lead  will  ap- 
pear in  appreciable  quantity  in  the  radium.  A  more  suitable 
substance  with  which  to  attack  the  question  would  be  radium  F 
(radiotellurium)  or  radiolead  (radium  D). 


CONSTITUTION  OF  THE  RADIOELEMENTS 

The  view  that  the  a  particle  is  a  helium  atom  suggests  that 
the  atoms  of  uranium  and  radium  are  built  up  in  part  of  atoms  of 
helium.  If  the  final  product  of  radium  is  lead,  the  radium  atom 
could  thus  be  represented  by  the  equation,  Ra  =  Pb  •  He^  while 
Ur  =  Pb-  ffes. 

It  must  be  borne  in  mind,  however,  that  these  compounds  of 
helium  are  very  different  from  ordinary  molecular  compounds. 
Both  radium  and  uranium  behave  as  elementary  substances, 
which  cannot  be  broken  up  by  the  application  of  physical  or 
chemical  forces  at  our  command.  These  substances  spontane- 
ously break  up  at  a  rate  that  is  independent  of  known  agencies,  and 
the  disintegration  is  accompanied  by  the  expulsion  of  a  helium 
atom  with  enormous  velocity.  The  energy  liberated  in  the  form 
of  the  kinetic  energy  of  the  expelled  helium  atoms  is  of  quite  a 
different  order  from  that  observed  in  molecular  reactions,  being  at 
least  one  million  times  as  great  as  that  released  in  the  most  vio- 
lent chemical  combinations.  It  seems  probable  that  the  helium 
atoms  are  in  very  rapid  motion  within  the  atoms  of  uranium 
and  radium,  and  for  some  reason  escape  from  the  atoms  with  the 
velocities  which  they  possessed  in  their  orbits.  The  forces  that 
hold  the  helium  atoms  in  place  in  the  atom  of  the  radioelements 
are  so  strong  that  no  means  at  our  disposal  are  able  to  effect 
their  separation. 

It  seems  probable  that  the  a  particles  from  thorium  and  actin- 
ium are  also  helium  atoms,  so  that  these  substances  must  also 
be  considered  as  compounds  of  some  unknown  substances  with 

13 


194  RADIOACTIVE   TRANSFORMATIONS 

helium.  Five  a  ray  products  are  known  to  exist  in  thorium, 
and  this  would  make  the  atomic  weight  of  the  residue  of  the 
thorium  atom  232.5  —  5x4,  or  212.5.  The  nearest  known 
atomic  weight  to  this  is  that  of  bismuth,  viz.  208,  and  if 
thorium  should  lose  six  a  particles  instead  of  five,  the  atomic 
weight  of  the  residue  would  be  very  nearly  that  of  bismuth.  This 
substance  also  fulfils  the  condition  required  for  a  transformation 
product  of  radioactive  substances,  for  it  is  found  in  radioactive 
minerals,  although  only  in  small  amount  compared  with  that 
of  lead  in  the  old  uranium  minerals,  -where  little  thorium  is 
present. 

It  thus  appears  that  helium  plays  a  most  important  part  in  the 
constitution  of  the  radioelements,  and  it  is  not  unlikely  that 
helium  as  well  as  hydrogen  may  ultimately  prove  to  be  one  of 
the  more  elementary  units  of  which  the  heavy  atoms  are  built  up. 
In  this  connection  it  may  prove  more  than  a  coincidence  that  a 
number  of  the  atomic  weights  of  the  elements  differ  by  nearly 
four  units  or  by  multiples  of  four  units. 

Many  of  the  primary  radioactive  minerals  were  undoubtedly 
deposited  at  the  surface  of  the  earth  100  to  1000  million  years 
ago,  and  since  that  time  have  been  undergoing  slow  trans- 
formation. There  is  no  evidence  at  hand  that  this  process  of 
degradation  of  matter  is  reversible  under  ordinary  conditions 
at  the  surface  of  the  earth.  It  seems,  however,  reasonable  to 
suppose  that  under  some  conditions,  existing  possibly  early  in 
the  earth's  history,  the  converse  process  took  place,  and  that  the 
heavy  atoms  were  built  up  from  the  lighter  and  more  elementary 
substances. 

It  may  happen  that  the  conditions  for  the  formation  of  heavy 
atoms  may  be  found  at  the  high  pressures  and  temperatures 
existing  deep  in  the  earth.  It  has  been  suggested  to  me  by  Dr. 
Barrel,  of  Yale  University,  that  the  gradual  building  up  of  the 
heavy  and  more  complex  atoms  of  matter  may  be  slowly  taking 
place  m  the  interior  of  the  earth,  and  that  this  might  possibly 
account  for  the  undoubtedly  high  density  of  the  matter  in  the 
interior  of  the  earth,  and  also  for  the  gradual  shrinking  of  the 
earth  as  a  whole. 


THE   PRODUCTION   OF  HELIUM 


195 


While  such  suggestions  are  at  present  highly  speculative,  it 
appears  not  unreasonable  to  suppose  that  the  formation  of  the 
radioactive  matter  may  still  be  in  progress  deep  in  the  earth, 
and  that  the  radioactive  deposits  found  at  the  surface  to-day 
have  been  forced  up  from  below  in  past  ages. 


OF  THE 


' 


UNIVERSITY 

OF 

'FORN\^ 


CHAPTER  IX 
RADIOACTIVITY  OF  THE  EARTH   AND   ATMOSPHERE 

WE  shall  in  this  chapter  briefly  discuss  the  present  state  of 
our  knowledge  of  the  radioactive  condition  of  the  earth  and 
atmosphere,  and  the  possible  bearing  of  the  facts  so  far  obtained 
on  the  problems  of  the  electrical  state  of  the  atmosphere  and  on 
the  internal  heat  of  the  earth. 

ATMOSPHERIC  RADIOACTIVITY 

The  remarkable  development  during  the  last  few  years  of  our 
knowledge  of  the  radioactive  and  electrical  state  of  the  atmos- 
phere is  one  of  unusual  interest,  and  although  the  interval  for 
investigation  has  been  short,  a  great  deal  of  new  and  important 
information  has  been  accumulated. 

Nearly  a  century  ago,  Coulomb  and  others  drew  attention  to 
the  fact  that  a  charged  conductor  placed  inside  a  closed  vessel 
lost  its  charge  more  rapidly  than  could  be  explained  by  the  con- 
duction of  electricity  along  the  insulating  support.  This  was 
thought  by  Coulomb  to  be  due  to  the  molecules  of  air  receiving 
a  charge  from  contact  with  the  charged  rod  and  then  being 
repelled  from  it.  As  early  as  1850,  Matte ucci  observed  that  the 
rate  of  loss  of  charge  was  independent  of  the  potential  of  the 
charged  body.  By  using  insulators  of  quartz  rods  of  different 
lengths  and  cross  section,  Boys  in  1889  came  to  the  conclusion 
that  the  loss  of  charge  could  not  be  explained  by  the  imperfect 
insulation  of  the  supports. 

Shortly  after  science  had  become  familiar  with  the  ionization 
of  gases  by  X-rays  and  uranium  rays,  the  question  of  the  cause 
of  this  loss  of  charge  was  independently  attacked  by  Geitel1 
and  C.  T.  R.  Wilson,2  using  specially  designed  electroscopes  to 

1  Geitel:  Physik.  Zeit.,  ii,  p.  116  (1900). 

2  Wilson:  Proc.  Camb.  Phil.  Soc. :   xi,  p.  32  (1900);  Proc.  Roy.  Soc.,  Ixviii, 
p.  151  (1901). 


RADIOACTIVITY   OF  THE   EARTH  197 

measure  the  rate  of  discharge  of  a  charged  body  inside  a  closed 
vessel.  Both  came  to  the  conclusion  that  the  gradual  loss  of 
charge  was  mainly  due  to  an  ionization  of  the  air  inside  the 
closed  vessel.  Above  a  certain  voltage,  the  rate  of  discharge 
was  independent  of  the  voltage,  a  result  to  be  expected  if  the 
ionization  was  very  weak.  It  was  at  first  thought  that  this 
ionization  in  the  gas  was  spontaneous  and  a  property  of  the 
gas  itself,  but  later  work  has  modified  this  conclusion.  It  is 
now  certain  that  a  large  part  of  the  ionization  observed  in  a 
clean  metal  vessel  results  mainly  from  the  emission  of  ionizing 
radiations  from  its  walls.  A  part  is  due  to  a  very  penetrating 
radiation  of  the  7  ray  type  which  is  everywhere  present  on  the 
surface  of  the  earth.  The  amount  of  ionization  of  a  gas  inside  a 

o 

closed  vessel  depends  on  the  nature  and  pressure  of  the  gas  and 
of  the  material  of  the  vessel.  In  most  cases  the  ionization  falls 
off  nearly  proportionally  with  the  pressure,  and  is  approximately 
proportional  to  the  density  of  the  gas.  Both  of  these  results 
are  to  be  expected  if  the  ionization  observed  is  due  to  radiations 
from  the  walls  or  to  a  penetrating  type  of  radiation  passing 
from  the  outside  through  the  material  of  the  vessel. 

It  must  be  borne  in  mind  that  the  natural  ionization  observed 
in  closed  vessels  is  extraordinarily  minute,  and  special  precau- 
tions are  usually  necessary  to  measure  it  with  accuracy.  As- 
suming that  the  ionization  in  a  small  silvered  glass  vessel  was 
uniform  throughout  its  volume,  C.  T.  R.  Wilson  found  that  not 
more  than  30  ions  were  produced  per  second  per  cubic  centi- 
metre of  the  enclosed  air.  In  a  vessel  of  one  litre  capacity,  the 
number  of  ions  produced  per  second  would  be  30,000,  or  only 
about  one  third  of  the  total  number  of  ions  produced  in  air  by 
a  single  a  particle  emitted  from  radium.  The  expulsion  of  a 
single  a  particle  per  second  from  the  walls  of  the  vessel  would 
thus  more  than  account  for  the  ionization  observed. 

After  examining  the  discharge  of  electricity  produced  by  air 
in  closed  vessels,  Elster  and  Geitel  turned  their  attention  to  the 
external  air.  They  found  that  a  charged  body  freely  exposed  to 
the  open  air  lost  its  charge  far  more  rapidly  than  when  placed  in 
a  small  closed  vessel.  Both  positive  and  negative  electricity  is 


198  RADIOACTIVE   TRANSFORMATIONS 

discharged,  but  generally  at  unequal  rates,  a  positively  charged 
body  losing  its  charge  somewhat  more  slowly  than  a  negatively 
charged  one.  The  ionization  of  the  open  air  was  examined  by 
means  of  a  portable  electroscope.  An  insulated  wire  gauze  was 
connected  to  the  charged  electroscope,  and  the  rate  of  loss  of 
charge  of  the  electroscope  was  taken  as  a  comparative  measure 
of  the  number  of  ions  in  the  air. 

In  the  course  of  their  experiments  on  closed  vessels,  Elster 
and  Geitel  noted  that  the  rate  of  discharge  increased  for  several 
hours  after  the  introduction  of  fresJa^jn?-  Such  a  result  was 
known  to  occur  when  the  radium  or  thorium  emanation  was 
mixed  with  the  air.  This  led  them  to  try  a  bold  experiment 
to  see  if  it  were  possible  to  extract  a  radioactive  substance 
from  the  atmosphere.  The  writer  had  shown  that  a  negatively 
charged  wire  exposed  in  the  presence  of  the  thorium  emanation 
became  strongly  active.  This  experiment  suggested  the  method 
of  attacking  the  question.1  A  long  wire  was  suspended  on 
insulating  supports  outside  the  laboratory  and  charged  nega- 
tively to  a  high  potential  by  means  of  a  static  machine.  After 
some  hours  the  wire  was  removed  and  coiled  round  the  top  of 
an  electroscope.  There  was  an  undoubted  increase  in  its  rate 
of  discharge,  showing  that  the  wire  possessed  the  new  property 
of  ionizing  the  gas.  The  effect  died  away  after  a  time,  and  was 
small  after  a  few  hours'  interval. 

Further  experiments  showed  that  the  wire  had  been  made 
temporarily  radioactive  by  exposure  to  the  open  air.  The 
amount  of  activity  observed  was  independent  of  the  nature  of 
the  material  of  the  wire,  and  in  this  respect  the  activity  behaved 
quite  similarly  to  the  excited  activity  imparted  to  bodies  in  the 
presence  of  the  radium  and  thorium  emanations. 

The  active  matter  could  be  dissolved  from  the  wire  by 
rubbing  it  with  leather  soaked  in  ammonia.  In  this  way 
an  active  substance  was  obtained  capable  of  affecting  a  pho- 
tographic plate  through  .1  mm.  of  aluminium  and  of  produc- 
ing weak  phosphorescence  on  a  screen  of  platinocyanide  of 
barium. 

i  Elster  and  Geitel:  Physik.  Zeit.,  iii,  p.  76  (1901). 


RADIOACTIVITY   OF   THE   EARTH  199 

Rutherford  and  Allan l  showed  that  similar  activity  could  be  ob- 
tained from  the  open  air  in  Montreal.  The  radiations  consisted 
of  a  and  /3  rays,  the  former  being  responsible  for  most  of  the 
ionization  observed  with  bare  wires.  The  activity  of  a  wire  made 
active  by  exposure  to  the  atmosphere  decayed  at  about  the  same 
rate  as  that  of  a  wire  made  active  by  exposure  to  the  radium 
emanation. 

Bumstead  and  Wheeler  2  examined  the  radioactive  state  of  the 
air  at  New  Haven,  and  from  a  comparison  of  the  rate  of  decay 
of  the  active  wire  with  that  of  a  wire  made  active  by  exposure 
to  the  radium  emanation,  showed  conclusively  that  the  activity 
observed  in  the  air  in  that  locality  was  mainly  due  to  the  radium 
emanation.  A  wire  made  active  in  the  open  air  showed  the 
initial  rapid  drop  of  activity  due  to  radium  A,  and  the  curve 
of  decay  was  identical  with  that  due  to  the  excited  activity  of 
radium.  An  emanation  was  obtained  by  boiling  the  soil  and 
surface  water  at  New  Haven,  which  decayed  at  the  same  rate  as 
the  radium  emanation. 

By  exposing  wires  for  several  days  in  the  open  air,  Bum- 
stead  3  also  observed  that,  after  the  excited  activity  due  to  the 
radium  emanation  had  disappeared,  a  part  of  the  activity  de- 
cayed much  more  slowly.  This  residual  activity  decayed  at  the 
same  rate  as  the  excited  activity  from  thorium,  showing  conclu- 
sively that  the  thorium  as  well  as  the  radium  emanation  was 
present  in  the  air.  Dadourian4  showed  that  the  soil  at  New 
Haven  was  impregnated  with  the  thorium  emanation.  A  hole 
was  dug  in  the  ground  and  the  top  closed.  A  negatively 
charged  wire  was  exposed  in  the  hole,  and  on  removal  it  was 
found  to  show  activity,  which  disappeared  at  the  characteristic 
rate  of  the  excited  activity  of  thorium. 

Such  results  show  that  the  soil  at  New  Haven  must  contain 
quite  appreciable  quantities  both  of  thorium  and  radium.  Since 
the  thorium  emanation  has  a  very  short  life,  it  is  only  able  to 
diffuse  into  the  open  air  from  a  small  depth  of  soil.  The  radium 

1  Rutherford  and  Allan  :  Phil.  Mag.,  Dec.,  1902. 

2  Bumstead  and  "Wheeler:  Amer.  Journ.  Science,  Feb.,  1904. 

3  Bumstead:  Amer.  Journ.  Sci.,  July,  1904. 

4  Dadourian:  Amer.  Journ.  Sci.,  Jan.,  1905. 


200  RADIOACTIVE   TRANSFORMATIONS 

emanation,  which  has  a  much  longer  life,  is  able  to  emerge  from 
a  much  greater  depth. 

In  the  meantime,  C.  T.  R.  Wilson  1  had  found  that  rain  was 
radioactive.  Rain  water  was  collected  after  a  shower,  and 
rapidly  evaporated  to  dryness  in  a  platinum  dish,  which  was 
then  placed  under  an  electroscope.  The  activity  was  found  to 
decay  to  half  value  in  about  30  minutes. 

Wilson  in  England,  S.  J.  Allan  and  McLennan  in  Canada, 
independently  showed  that  freshly  fallen  snow  possessed  a  like 
property.  The  activity  of  snow,  like  that  of  rain,  falls  to  half 
value  in  30  minutes.  This  rate  of  decay  is  nearly  the  same  as 
that  observed  for  the  excited  activity  of  radium,  several  hours 
after  removal  of  the  emanation.  Such  a  result  suggests  that 
the  carriers  of  radium  B  and  radium  C  become  attached,  prob- 
ably by  diffusion,  to  the  water  drops  or  snowflakes  in  their 
passage  through  the  air.  On  evaporation,  the  active  matter 
remains  behind.  A  heavy  fall  of  rain  or  a  snowstorm  must  thus 
act  as  a  means  of  temporarily  removing  a  proportion  of  the 
radium  B  and  C  always  present  in  the  air. 

Elster  and  Geitel  found  that  the  air  in  confined  spaces,  such 
as  caves  and  cellars,  was  abnormally  radioactive,  and  showed 
strong  ionization.  To  show  that  these  effects  did  not  result 
from  stagnant  air  alone,  Elster  and  Geitel  confined  a  large 
volume  of  air  in  an  old  steam  boiler,  but  did  not  observe  any 
increase  of  the  ionization  with  time.  Other  experiments  showed 
that  the  increased  radioactivity  in  confined  spaces,  in  contact 
with  the  earth,  was  due  to  the  gradual  storing  of  the  radium 
emanation  which  diffused  through  the  soil.  In  order  to  throw 
light  on  this  question,  Elster  and  Geitel 2  placed  a  pipe  several 
feet  deep  in  the  earth,  and  by  means  of  a  pump  sucked  up  some 
of  the  air  imprisoned  in  the  capillaries  of  the  soil.  This  was 
found  to  be  strongly  active,  and  its  activity  decayed  at  about 
the  same  rate  as  that  of  air  mixed  with  the  radium  emanation. 

Similar  results  were  observed  by  Ebert  and  Ewers  3  for  the 

1  Wilson :  Proc.  Camb.  Phil.  Soc.,  xi,  p.  428  (1902)  ;  xii,  p.  17  (1903). 

2  Elster  and  Geitel :  Physik.  Zeit.,  iii,  p.  574  (1902). 
8  Ebert  and  Ewers  :  Physik.  Zeit.,  iv,  p.  162  (1902). 


RADIOACTIVITY   OF  THE   EARTH  201 

air  removed  from  the  soil  at  Munich.  Such  results  show  con- 
clusively that  small  quantities  of  radium  are  everywhere  distrib- 
uted throughout  the  surface  soil  of  the  earth.  J.  J.  Thomson, 
Adams,  and  others  examined  the  water  obtained  from  deep  wells 
and  springs  in  England,  and  found  that  in  some  cases  the  water 
contained  considerable  quantities  of  the  radium  emanation,  and 
in  a  few  cases  a  trace  of  radium  itself. 

In  the  last  few  years,  a  very  large  amount  of  work  has  been 
done  in  examining  the  waters  and  sediments  of  mineral  and 
hot  springs  for  the  presence  of  radioactive  matter.  H.  S.  Allan 
and  Lord  Blythswood  found  that  the  hot  springs  at  Bath  and 
Buxton  contained  appreciable  quantities  of  a  radioactive  emana- 
tion. This  was  confirmed  by  Strutt,  who  found  that  not  only 
was  the  radium  emanation  contained  in  the  issuing  water,  but 
that  the  mud  deposited  by  the  springs  contained  traces  of 
radium.  It  is  of  interest  to  note  that  helium  has  been  observed 
amongst  the  gases  evolved  by  these  springs,  and  it  would  appear 
probable  that  the  waters  in  their  passage  to  the  earth  pass 
through  a  deposit  of  radioactive  minerals. 

Himstedt  found  the  radium  emanation  in  the  thermal  springs 
at  Baden  Baden,  while  Elster  and  Geitel  found  also  small  traces 
of  radium  in  the  mud  deposited  by  them.  A  large  number  of 
springs  have  been  examined  by  different  observers  in  England, 
Germany,  France,  Italy,  and  the  United  States,  and  in  nearly 
all  cases  the  radium  emanation  has  been  found  in  the  water, 
often  in  easily  measurable  amount.  Elster  and  Geitel  found 
that  the  mud  or  "  fango  "  deposited  from  the  hot  springs  at 
Battaglia,  Italy,  was  abnormally  radioactive,  and  a  close  ex- 
amination showed  that  the  activity  was  due  to  radium.  They 
calculated  that,  weight  for  weight,  it  contains  almost  one  thou- 
sandth of  the  radium  to  be  obtained  from  the  Joachimsthal 
pitchblende. 

While  the  activity  of  most  of  the  waters  of  hot  springs  is  due 
in  most  cases  to  the  presence  of  radium  or  its  emanation,  Blanc l 
has  observed  one  notable  exception  in  which  the  activity  is  mainly 
due  to  thorium.  The  sediment  of  the  waters  at  Salins-Moutiers 

i  Blanc  :  Phil.  Mag.,  Jan.,  1905. 


202  RADIOACTIVE   TRANSFORMATIONS 

was  abnormally  active,  and  was  found  to  give  off  considerable 
quantities  of  the  thorium  emanation.  Blanc,  however,  was 
unable  to  detect  analytically  the  presence  of  thorium,  although, 
from  the  amount  of  emanation  given  off,  a  considerable  quantity 
should  have  been  present.  It  seems  not  unlikely  that  the  activ- 
ity observed  was  due  not  to  the  primary  substance  thorium,  but 
to  its  product,  radiothorium,  which  was  discovered  by  Hahn 
(see  page  68).  This  would  give  rise  to  thorium  X  and  the  tho- 
rium emanation,  but  would  be  present  in  too  minute  a  quantity 
to  be  determined  chemically. 

Elster  and  Geitel  observed  that  natural  carbonic  acid  obtained 
from  great  depths  of  old  volcanic  soil  contained  the  radium 
emanation,  while  McLennan  and  Burton  found  considerable 
quantities  of  radium  emanation  in  the  petroleum  from  a  deep 
well  in  Ontario,  Canada. 

In  most  cases  where  spring  water  comes  from  great  depths, 
and  especially  if  the  water  is  hot*  radioactive  matter  is  found  to 
be  present  in  abnormal  amount  compared  with  that  found  in  the 
soil  itself.  Such  a  result  is  not  unexpected,  for  water,  and  par- 
ticularly hot  water,  would  tend  to  dissolve  traces  of  radioactive 
matter  in  the  strata  through  which  it  passes,  and  also  to  become 
impregnated  with  the  radium  emanation.  In  special  cases,  it 
may  happen  that  the  water  has  passed  through  a  deposit  of 
radioactive  minerals,  and  in  such  a  case,  a  very  strong  activity 
is  to  be  expected. 

Elster  and  Geitel  have  made  an  extensive  examination  of  dif- 
ferent soils  for  radioactivity,  and  have  found  traces  of  radioac- 
tive matter  in  nearly  every  case.  The  activity  is  most  marked 
in  clayey  soils,  and  is  apparently  due  in  many  cases  to  the  pres- 
ence of  small  traces  of  radium.  The  observations  as  a  whole 
show  clearly  that  radioactive  matter  is  extraordinarily  diffused  in 
nature,  and  it  is  difficult  to  find  any  substance  that  does  not 
contain  a  minute  trace  of  radium.  It  does  not  appear  likely 
that  uranium  or  radium  differ  in  this  respect  from  the  inactive 
elements. 

The  presence  of  radium  can  be  noted  by  the  electric  test, 
where  chemical  analyses  would  fail  to  detect  the  presence  of 


RADIOACTIVITY   OF   THE   EARTH  203 

rare  inactive  elements,  although  they  may  exist  in  considerably 
greater  quantity  than  the  radium  itself.  On  general  grounds, 
such  a  wide  diffusion  of  radioactive  matter  is  not  surprising,  for 
the  soil  of  the  earth  at  any  point  should  contain  a  fairly  thorough 
admixture  of  a  great  majority  of  the  elements  found  in  the  earth, 
the  rare  elements  being  present  in  only  minute  proportion. 

There  can  be  no  reasonable  doubt  that  the  radioactive  matter 
observed  in  the  atmosphere  is  mainly  due  to  the  emanation  of 
radium  and  its  transformation  products,  and  probably  in  some 
localities  to  traces  of  the  emanation  of  thorium  and  actinium. 
The  supply  of  radioactive  matter  in  the  atmosphere  is  kept  up 
mainly  by  the  diffusion  and  transpiration  of  the  emanations 
through  the  soil,  while  no  doubt  a  part  is  supplied  by  the  action 
of  springs  and  by  the  release  of  imprisoned  gases. 

On  account  of  its  comparatively  slow  rate  of  change,  it  is  to 
be  expected  that  the  amount  of  the  emanation  of  radium  will  pre- 
dominate over  the  other  emanations  in  the  atmosphere,  for  the 
short  life  of  the  emanations  of  actinium  and  thorium  prevent 
their  reaching  the  surface  from  any  appreciable  depth.  While  it 
is  probable  that  the  supply  of  emanation  from  the  earth  to  the 
atmosphere  varies  in  different  localities,  the  action  of  winds  and 
air  currents  generally  should  tend  to  distribute  the  emanation 
from  point  to  point  and  to  make  its  distribution  more  uniform. 

All  observers  have  noted  that  the  amount  of  excited  activity 
to  be  obtained  under  definite  conditions  from  the  atmosphere  is 
very  variable,  and  often  alters  considerably  during  a  single  day. 
Elster  and  Geitel  made  a  detailed  examination  of  the  effect 
of  meteorological  conditions  on  the  amount  of  active  matter  in 
the  atmosphere.  The  experiments  were  made  at  Wolfenbiittel, 
Germany,  and  were  continued  for  twelve  months.  On  an  aver- 
age, the  amount  of  active  matter  increased  with  a  lowering  of 
the  temperature.  Below  0°  C.,  the  average  was  1.44  times  as 
great  as  above  0°  C.  A  falling  barometer  increases  the  amount 
of  active  matter.  The  effect  of  a  change  of  pressure  is  intelli- 
gible when  it  is  remembered  that  a  lowering  of  the  pressure 
tends  to  cause  the  emanation  in  the  capillaries  of  the  soil  to  be 
drawn  to  the  surface. 


204  RADIOACTIVE   TRANSFORMATIONS 

If  the  emanation  observed  in  the  atmosphere  is  entirely  drawn 
from  the  soil,  the  amount  in  the  air  over  the  sea  in  mid-ocean 
should  be  much  smaller  than  on  land,  for  the  water  will  not 
allow  the  emanation  to  escape  from  the  earth's  crust  into  the 
atmosphere.  Observations  so  far  made  indicate  that  the  amount 
of  active  matter  in  the  air  falls  off  near  the  sea.  For  example, 
the  amount  on  the  Baltic  coast  was  found  by  Elster  and  Geitel 
to  be  only  about  one  third  of  that  found  inland,  but  no  systematic 
examination  has  yet  been  made  of  the  amount  of  active  matter 
in  the  atmosphere  at  great  distances  from  land. 

AMOUNT  OP  THE  RADIUM  EMANATION  IN  THE 
ATMOSPHERE 

Most  of  the  experiments  on  the  amount  of  active  matter  in 
the  air  have  been  qualitative  in  character,  but  it  is  obviously 
important  to  obtain  some  idea  of  the  amount  of  the  radium 
emanation  present  in  the  atmosphere.  Since  the  amount  in  the 
atmosphere  is  kept  up  by  a  constant  supply  of  fresh  emanation 
from  the  earth,  it  is  convenient  to  express  the  amount  of  radio- 
active matter  in  the  atmosphere  in  terms  of  the  amount  of  freely 
emanating  radium  bromide  which  is  required  to  keep  up  the 
supply. 

Some  interesting  experiments  in  this  direction  have  recently 
been  made  by  A.  S.  Eve 1  at  Montreal.  The  radioactive  state  of 
the  air  in  the  neighborhood  of  Montreal  appears  to  be  normal, 
and  the  number  of  ions  present  per  cubic  centimetre  of  the  out- 
side air  is  about  the  same  as  that  observed  at  different  localities 
in  Europe. 

Some  experiments  were  first  made  in  a  large  iron  tank  in  the 
Engineering  Building  of  Me  Gill  University.  This  tank  was 
8.08  metres  high  and  1.52  metres  square,  with  a  total  volume  of 
18.7  cubic  metres.  In  order  to  determine  the  amount  of  ex- 
cited activity  to  be  obtained  from  the  tank,  a  long  insulated 
wire  was  suspended  in  its  centre,  and  kept  at  a  constant  poten- 
tial of  —10,000  volts  for  three  hours.  The  wire  was  then  rapidly 

i  A.  S.  Eve :  Phil.  Mag.,  July,  1905. 


RADIOACTIVITY   OF   THE   EARTH  205 

removed,  and  coiled  round  a  frame  attached  to  an  electroscope. 
The  rate  of  fall  of  the  gold  leaves  then  served  as  a  measure  of 
the  amount  of  active  matter  deposited  on  the  wire. 

A  similar  experiment  was  then  made  in  a  small  zinc  cylinder 
of  volume  76  litres.  The  emanation  derived  from  2x1 0~4  milli- 
grams of  radium  bromide  was  introduced  into  the  cylinder  and 
mixed  with  the  air.  The  excited  activity  was  concentrated,  as 
before,  on  a  negatively  charged  wire,  and  after  removal  was 
measured  with  the  same  apparatus  employed  in  the  first  experi- 
ment. Knowing  the  rate  of  discharge  of  the  electroscope  for 
the  active  deposit  obtained  from  a  known  quantity  of  radium,  a 
comparison  of  the  results  obtained  with  the  large  tank  at  once 
gave  the  amount  of  emanation  present  in  it.  In  this  way,  it 
was  calculated  that  one  cubic  kilometre  of  the  air,  containing 
the  same  amount  of  emanation  per  unit  volume  as  that  observed 
in  the  large  tank,  was  equivalent  to  the  emanation  supplied  by 
.49  grams  of  pure  radium  bromide. 

The  tank  in  these  experiments  was  in  free  communication 
with  the  open  air,  and  the  amount  of  the  excited  activity  was 
unaltered  when  the  air  from  the  room  was  forced  through  the 
tank.  It  thus  seems  reasonable  to  suppose  that  the  air  within 
the  tank  contained  the  same  amount  of  emanation  per  unit 
volume  as  the  outside  air.  No  radioactive  matter  had  ever  been 
introduced  into  the  building  where  the  tank  was  used,  and,  as 
we  shall  see  later,  the  rate  of  production  of  ions  per  cubic  centi- 
metre of  the  tank  was  lower  than  that  ever  previously  recorded. 

In  order  to  verify  this  point,  however,  experiments  were 
made  on  another  large  zinc  cylinder  placed  on  the  College 
Campus,  with  its  ends  in  free  communication  with  the  air.  The 
active  deposit  was  collected  on  a  wire  suspended  along  the  axis 
of  the  tank,  and  tested  as  before.  The  average  amount,  how- 
ever, was  found  to  be  only  from  one  third  to  one  fourth  of  that 
observed  for  an  equal  volume  of  the  large  tank.  No  adequate 
cause  could  be  assigned  for  this  discrepancy  between  the  results 
for  the  experiments  in  the  iron  tank  and  in  the  zinc  cylinder, 
unless  the  charged  wire  is  unable  for  some  reason  to  collect  all 
the  active  deposit  from  the  cylinder  placed  in  the  open  air. 


206  RADIOACTIVE   TRANSFORMATIONS 

On  certain  assumptions  we  can  form  a  rough  estimate  of  the 
amount  of  radium  emanation  existing  in  the  atmosphere.  Sup- 
pose the  emanation  is  uniformly  distributed  in  a  spherical  layer 
10  kilometres  deep  around  the  earth,  and  that  the  emanation  per 
cubic  kilometre  is  uniform,  and  equal  to  that  observed  at  Mon- 
treal. The  surface  of  the  earth  is  about  5  x  10 8  square  kilo- 
metres, and  the  volume  of  the  shell  10  kilometres  thick  is  5  x  10  9 
cubic  kilometres.  Taking  the  estimated  value,  .49  grams  per 
cubic  kilometre,  found  from  experiments  on  the  large  tank, 
the  amount  of  emanation  in  the  atmosphere  corresponds  to 
2.5  x  10  9  grams  or  2,460  tons  of  radium  bromide. 

Now,  about  three  quarters  of  the  earth  is  covered  with  water, 
through  which  no  emanation  can  escape  to  the  surface.  If 
the  emanation  arises  from  the  land  alone,  the  amount  is  thus 
reduced  to  about  one  quarter  of  this,  or  610  tons.  Taking  the 
value  obtained  from  measurements  in  the  cylinder  in  the  outside 
air,  the  amount  is  found  to  be  about  170  tons. 

Several  observers  have  shown  that  the  amount  of  excited 
activity  present  in  the  air  at  high  altitudes  is  equal  to,  if  not 
greater  than,  the  amount  observed  on  the  plains.  It  thus  ap- 
pears probable  that  in  supposing  that  the  emanation  is  distrib- 
uted to  an  average  height  of  10  kilometres  in  the  atmosphere  we 
are  well  within  the  truth.  Until  a  very  complete  radioactive 
survey  of  the  atmosphere  has  been  made,  such  calculations  are 
of  necessity  somewhat  uncertain,  but  they  certainly  serve  to  give 
the  right  order  of  magnitude  of  the  quantities  involved. 

Since  the  emanation  is  half  transformed  in  about  four  days,  it 
cannot  diffuse  into  the  air  from  any  great  depth  in  the  earth,  so 
that  the  main  supply  of  the  emanation  must  come  from  a  super- 
ficial layer  of  the  earth  not  many  metres  in  thickness.  A  part 
of  the  supply  is  probably  due  to  deep  seated  springs,  which  may 
bring  up  the  emanation  from  greater  depths,  but  the  amount  so 
supplied  is  probably  small  compared  with  that  escaping  directly 
through  the  pores  of  the  soil. 

We  thus  arrive  at  the  important  conclusion  that  a  very  con- 
siderable quantity  of  radium,  measured  by  hundreds  of  tons,  is 
distributed  over  the  earth  within  a  few  metres  of  its  surface. 


RADIOACTIVITY   OF  THE   EARTH  207 

It  is  for  the  most  part,  however,  distributed  in  such  infinitesimal 
quantities  that  its  presence  can  be  detected  only  by  the  aid  of 
the  electric  method. 

Eve  (loc.  cit.)  found  that  a  wire  about  one  millimetre  in  diam- 
eter charged  to  —10,000  volts,  and  suspended  about  20  feet  above 
the  ground,  was  only  able  to  collect  the  active  deposit  from  the 
air  in  a  cylindrical  volume  of  radius  lying  between  40  and  80  cms. 
This  collecting  distance  is  small  compared  with  that  to  be  ex- 
pected for  such  a  high  voltage,  for  the  writer  has  shown  that  the 
positively  charged  carriers  of  the  active  deposit  of  radium  and 
of  thorium  travel  in  an  electric  field  at  about  the  same  velocity 
as  the  ion,  i.  e.,  they  move  with  a  velocity  of  about  1.4  cms.  per 
second  under  a  potential  gradient  of  one  volt  per  cm.  It  seems 
probable  that  the  carriers  of  the  active  deposit,  which  must 
remain  suspended  a  long  time  in  the  atmosphere,  adhere  to  the 
comparatively  large  dust  nuclei  always  present  in  the  air,  and 
consequently  move  very  slowly  in  an  electric  field,  so  that  the 
carriers  can  only  be  drawn  in  from  the  immediate  vicinity  of  the 
charged  wire. 

THE  PENETRATING  RADIATION  AT  THE  EARTH'S 
SURFACE 

Since  the  radium  emanation  is  everywhere  present  in  the  sur- 
face of  the  earth  and  in  the  atmosphere,  its  transformation  prod- 
uct radium  C  must  give  rise  to  7  rays,  and  these  rays  must 
come  in  all  directions  from  the  earth  and  atmosphere.  The 
presence  of  such  a  penetrating  radiation  at  the  earth's  surface 
was  independently  observed  by  McLennan l  and  H.  L.  Cooke,2 
in  Canada.  McLennan  worked  with  a  large  vessel,  and  observed 
that  the  ionization  of  the  air  inside  diminished  about  37  per 
cent  when  the  vessel  was  surrounded  by  a  thickness  of  25  cms. 
of  water.  Cooke  worked  with  a  small  brass  electroscope  of  about 
one  litre  capacity.  The  rate  of  discharge  of  the  electroscope 
fell  about  30  per  cent  when  completely  surrounded  by  a  lead 
screen  5  cms.  thick.  No  further  diminution  was  observed  by 

1  McLennan  :  Phys.  Rev.,  No.  4,  1903. 

2  Cooke:  Phil.  Mag.,  Oct.,  1903. 


208  RADIOACTIVE   TRANSFORMATIONS 

placing  a  ton  of  lead  around  the  apparatus.  The  radiation  is  of 
about  the  same  penetrating  power  as  the  7  rays  from  radium, 
and  can  be  observed  in  the  open  air  as  well  as  in  a  building. 
By  placing  blocks  of  lead  in  different  positions  in  regard  to  the 
electroscope,  it  was  found  that  the  radiation  came  about  equally 
from  all  directions,  and  was  the  same  during  the  day  as  at  night. 
Such  results  are  to  be  expected  if  the  penetrating  rays  come 
equally  from  the  radioactive  matter  distributed  in  the  earth  and 
atmosphere.  The  magnitude  of  the  ionizing  effect  due  to  the 
penetrating  rays  is,  however,  much  greater  than  that  due  to  the 
7  rays  from  the  amount  of  radium  emanation  in  the  atmosphere 
calculated  by  Eve.  It  seems  not  unlikely  that  these  penetrat- 
ing rays  may  be  given  out  by  matter  in  general  as  well  as  by 
radioactive  bodies. 

ELECTRICAL  STATE  OF  THE  ATMOSPHERE 

It  has  long  been  known,  from  observation  of  the  potential 
gradient  in  the  atmosphere,  that  the  upper  layers  of  the  atmos- 
phere are  generally  positively  charged  in  regard  to  the  earth. 
There  is  consequently  an  electric  field  always  acting  between  the 
earth  and  the  upper  atmosphere.  Since  there  is  a  distribution 
of  ions  in  the  lower  regions  of  the  atmosphere,  there  must  con- 
sequently be  a  steady  movement  of  negative  ions  upwards  and 
of  positive  ions  downwards  towards  the  earth.  Since  the  car- 
riers of  the  active  deposit  of  radium  have  a  positive  charge,  they 
must  tend  to  be  deposited  on  the  surface  of  the  earth.  Each 
blade  of  grass  and  each  leaf  must  consequently  be  coated  with 
an  invisible  deposit  of  radioactive  material.  A  hill  or  mountain 
top  tends  to  concentrate  the  earth's  field  at  that  point,  and  there 
should  be  a  greater  amount  of  active  matter  deposited  on  its 
surface  than  on  an  equal  area  on  the  plains.  This  is  in  agree- 
ment with  the  observations  of  Elster  and  Geitel,  who  found 
that  the  ionization  of  the  air  on  a  mountain  top  was  greater  than 
on  a  lower  level. 

A  large  number  of  observations  have  been  made  of  the  rela- 
tive number  of  ions  in  the  air  at  various  localities  under  differ- 
ent meteorological  conditions.  Many  experimenters  have  used 


RADIOACTIVITY   OF  THE   EARTH  209 

the  "  dissipation  apparatus "  constructed  by  Elster  and  Geitel. 
This  consists  of  an  open  wire  gauze  connected  with  an  electro- 
scope. The  rate  of  discharge  of  the  electroscope  is  separately 
observed  when  charged  positively  and  negatively.  While  this 
apparatus  has  proved  of  value  in  preliminary  work  on  the  ioniza- 
tion  of  the  atmosphere,  the  results  obtained  are  only  comparative, 
and  do  not  readily  lend  themselves  to  quantitative  calculations. 
The  effect  of  wind  is  very  marked  in  such  an  apparatus,  and  the 
rate  of  dissipation  is  always  higher  when  a  wind  is  blowing. 

A  very  useful  portable  instrument  for  determining  the  actual 
number  of  positive  and  negative  ions  per  cubic  centimetre  of 
the  air  has  been  devised  by  Ebert.1  By  means  of  a  fan  driven 
by  clockwork,  a  steady  current  of  air  is  drawn  between  two  con- 
centric cylinders.  The  inner  cylinder  is  insulated  and  connected 
with  a  direct  reading  electroscope.  The  length  of  the  cylinder 
is  so  adjusted  that  all  the  ions  in  the  air  are  drawn  to  the  elec- 
trodes in  their  passage  through  the  cylinder.  Knowing  the 
capacity  of  the  instrument,  the  velocity  of  the  current  of  air, 
and  the  constants  of  the  electroscope,  the  number  of  ions  per  c.c. 
of  the  air  can  easily  be  deduced.  When  the  inner  cylinder  is 
charged  positively,  the  rate  of  discharge  of  the  electroscope  is  a 
measure  of  the  number  of  negative  ions  in  the  air,  and  vice  versa. 

Measurements  by  Eberts  and  others  show  that  the  actual 
number  of  ions  per  c.c.  of  air  is  subject  to  considerable  fluctua- 
tions and  is  dependent  on  meteorological  conditions.  The  num- 
ber usually  varies  between  five  hundred  and  several  thousands, 
and  the  number  of  positive  ions  is  nearly  always  greater  than 
the  number  of  negative. 

Schuster2  observed  that  the  number  of  ions  per  c.c.  in  the  air 
in  Manchester  varied  between  2300  and  3700.  These  numbers 
give  the  equilibrium  number  of  ions  in  the  air  when  the  rate  of 
production  of  fresh  ions  is  balanced  by  the  rate  of  their  recom- 
bination. If  nv  n2  are  the  number  of  positive  and  negative  ions 
respectively  per  c.c.  of  air,  and  q  the  rate  of  production  per  c.c. 

1  Ebert:  Physik.  Zeit.,  ii,  p.  662  (1901);  Zeitschr.  f.  Luftschiff-fahrt,  iv,  Oct., 
(1902). 

2  Schuster :  Proc.  Manchester  Phil.  Soc.,  p.  488,  No.  12,  1904. 

14 


210  RADIOACTIVE   TRANSFORMATIONS 

per  second,  then  q  =  a  n^  n2,  where  a  is  the  coefficient  of  recom- 
bination of  the  ions.  By  a  slight  modification  of  the  apparatus 
of  Ebert,  Schuster  was  able  to  determine  the  value  of  a  for  the 
air  under  the  normal  conditions  of  experiment,  and  deduced  that 
the  value  of  q  in  Manchester  varied  between  12  and  39. 

The  apparatus  of  Ebert  was  designed  to  measure  the  number 
of  free  ions  in  the  air  which  have  the  same  mobility  as  the 
ions  produced  by  X-rays  or  the  radiations  from  active  bodies. 
The  velocity  of  the  ions  produced  in  air  have  been  directly 
measured  by  Mache  and  von  Schweidler.  The  positive  ion 
moves  1.02  cms.  per  second,  and  the  negative  1.25  cm.  per 
second,  for  a  potential  gradient  of  one  volt  per  cm.  These 
velocities  are  slightly  slower  than  those  observed  for  the  ions 
produced  in  dust-free  air  by  X-rays  or  the  rays  from  radioactive 
substances. 

In  addition  to  these  swiftly  moving  ions,  Langevin l  has 
shown  that  a  number  of  slowly  moving  ones  are  also  present, 
which  travel  too  slowly  in  an  electric  field  to  be  removed  by  the 
electric  field  used  in  the  apparatus  of  Ebert.  These  ions  move 
with  about  the  same  velocity  as  the  ions  observed  in  flame  gases 
some  distance  from  the  flame.  By  using  much  stronger  fields, 
Langevin  has  determined  the  number  of  these  heavy  ions  pres- 
ent in  the  air,  and  concludes  that  they  are  about  forty  times  as 
numerous  as  the  swiftly  moving  ones.  It  is  possible  that  these 
slowly  moving  ions  are  formed  by  the  deposition  of  water  round 
the  ion  to  form  a  minute  globule,  or  by  the  adherence  of  the  ion 
to  the  dust  which  is  always  present  in  the  air. 

Since  there  is  undoubtedly  a  continuous  production  of  ions 
in  the  air  near  the  earth,  it  is  a  matter  of  great  importance  to 
determine  the  cause  or  causes  of  this  ionization.  The  most 
obvious  cause  is  the  presence  of  radioactive  matter  in  the  atmos- 
phere. But  is  the  amount  present  capable  of  producing  the 
ionization  observed  ?  In  order  to  throw  light  on  this  important 
point,  Eve  (loc.  cit.)  made  the  following  experiment.  The  large 
iron  tank,  previously  described  on  page  204,  was  used.  An 
insulated  cylindrical  electrode  passed  down  the  centre  of  the 

1  Langevin  :  Comptes  rendus,  cxl,  p.  232  (1905). 


RADIOACTIVITY   OF   THE   EARTH  211 

tank  and  was  connected  to  an  electroscope.  The  electrode  was 
charged  to  a  sufficient  potential  to  obtain  the  saturation  current, 
which  is  a  measure  of  the  total  number  of  ions  produced  per 
second.  A  wire  charged  to  — 10,000  volts  was  then  suspended 
in  the  tank  and  the  active  deposit  collected  from  it  for  a  definite 
"time.  The  activity  imparted  to  the  wire  was  measured  imme- 
diately after  removal  with  an  electroscope. 

An  exactly  similar  set  of  experiments  was  then  made  with  a 
much  smaller  zinc  cylinder,  the  air  of  which  was  artificially  sup- 
plied with  the  radium  emanation  obtained  by  blowing  air  through 
a  radium  solution.  The  saturation  current  was  measured,  and 
also  the  amount  of  the  activity  imparted  to  a  central  electrode 
under  the  same  conditions  as  in  the  large  tank.  If  the  ioniza- 
tion  in  the  large  tank  is  due  entirely  to  the  presence  of  the 
radium  emanation  in  it,  then  the  ratio  of  the  saturation  currents 
in  the  two  tanks  should  be  equal  to  the  ratio  of  the  activities 
imparted  to  the  collecting  wires  under  the  same  experimental 
conditions.  This  must  obviously  be  the  case,  since  the  saturation 
current  serves  as  a  measure  of  the  amount  of  emanation  present, 
and  so  also  does  the  activity  imparted  to  the  collecting  wire. 

The  ratio  of  the  activity  on  the  collecting  wire  in  the  iron 
tank  to  that  in  the  emanation  cylinder  was  found  to  be  about  14 
per  cent  less  than  the  ratio  of  the  corresponding  saturation  cur- 
rents. Considering  the  difficulty  of  such  experiments,  the  agree- 
ment is  as  close  as  could  be  expected,  and  indicates  that  the 
greater  part,  if  not  all,  of  the  ionization  observed  in  the  iron 
tank  was  due  to  the  presence  of  the  radium  emanation. 

Since  there  was  every  reason  to  believe  that  the  air  in  the  tank 
contained  the  same  amount  of  emanation  as  the  outside  air,  this 
result  indicates  that  the  production  of  ions  in  the  outside  air 
is  mainly  due  to  the  radioactive  matter  contained  in  it.  Be- 
fore such  a  conclusion  can  be  considered  as  established,  experi- 
ments of  a  similar  character  must  be  made  in  various  localities. 
We  are,  in  any  case,  justified  in  assuming  that  the  radioactive 
matter  in  the  air  plays  a  very  important  part  in  the  produc- 
tion of  ions  observed  in  the  atmosphere  near  the  surface  of  the 
earth. 


212  RADIOACTIVE   TRANSFORMATIONS 

It  is  of  interest  to  record  that  Eve  found  the  number  of  ions 
produced  per  c.c.  per  second  in  the  iron  tank  to  be  9.8.  This  is 
the  smallest  rate  of  production  of  ions  yet  recorded  for  a  closed 
vessel.  Cooke  observed  a  value  as  low  as  20  for  a  well  cleaned 
brass  electroscope  of  about  one  litre  capacity. 

If  the  radioactive  matter  in  the  air  is  the  cause  of  its  ioniza- 
tion,  there  should  be  a  constant  proportion  between  the  rate  of 
production  of  ions  in  the  air  and  the  excited  activity  on  the  col- 
lecting wire.  The  data  so  far  collected  by  various  observers  ap- 
pear to  contradict  such  a  connection.  It  is  doubtful,  however, 
whether  the  measurements  actually  supply  the  data  required. 

There  seems  to  be  no  doubt  that  the  recombination  constant 
of  the  ions  depends  greatly  on  meteorological  conditions,  and  on 
the  freedom  of  the  air  from  nuclei.  The  variation  of  this  con- 
stant affects  the  equilibrium  number  of  ions  in  the  air  deter- 
mined by  the  apparatus  of  Ebert.  In  a  similar  way,  the 
excited  activity  imparted  to  a  charged  wire  in  the  open  air 
will  probably  depend  upon  atmospheric  conditions,  although  the 
amount  of  emanation  present  may  not  have  been  changed. 
Before  any  definite  conclusion  can  be  reached,  it  will  be  neces- 
sary to  take  all  these  factors  into  account.  A  large  number 
of  observations  have  been  made  in  Germany  on  the  effect  of 
meteorological  conditions  on  the  amount  of  dissipation  measured 
by  Elster  and  Geitel's  apparatus.  We  have  already  mentioned 
the  effect  of  a  rising  or  falling  barometer,  in  producing  well 
marked  variations  in  the  amount  of  active  matter  in  the  air. 
The  relation  between  potential  gradient  and  dissipation  has  been 
studied  by  Gockel  and  Zolss.  The  latter  finds  that  the  poten- 
tial gradient  varies  in  a  marked  manner  with  the  dissipation. 
A  high  potential  gradient  is  accompanied  by  a  low  value  of  the 
dissipation,  and  vice  versa.  A  similar  relation  between  the  poten- 
tial gradient  and  the  amount  of  ionization  determined  by  Ebert's 
apparatus  has  been  observed  by  Simpson  in  Norway.  Elster 
and  Geitel,  and  Zolss  have  shown  that  the  dissipation  increases 
with  the  temperature.  Simpson  found  that  at  Karasjoh  in  Nor- 
way, the  average  between  temperatures  of  10°  C.  and  15°  C.  was 
about  six  times  as  great  as  that  between  —40°  C.  and  —20°  C. 


RADIOACTIVITY   OF  THE   EARTH  213 

A  very  complete  series  of  observations  on  the  annual  varia- 
tion of  the  potential  gradient,  ionization,  and  dissipation,  was 
made  by  Simpson  l  at  Karasjoh,  in  Norway,  situated  within  the 
Arctic  circle,  in  latitude  69°.  These  results  are  of  special  in- 
terest, for  between  November  26  and  January  18  the  sun  did 
not  rise  above  the  horizon,  while  between  May  20  and  July  22 
the  sun  did  not  fall  below  the  horizon. 

The  absence  of  the  sun's  rays  apparently  had  no  marked 
effect  on  the  magnitude  of  the  quantities  measured.  There  was 
on  an  average  a  steady  rise  of  the  potential  gradient  between 
October  and  February,  and  a  steady  fall  of  the  ionization  during 
the  same  period.  Such  results  indicate  that  the  sun's  rays  have 
little  if  any  direct  effect  on  the  ionization  of  the  air. 

It  is  impossible  here  to  discuss  the  numerous  speculations 
that  have  been  advanced  to  account  for  the  presence  of  a  strong 
positive  charge  in  the  upper  atmosphere.  This  positive  charge 
must  be  steadily  supplied  from  some  source,  for  otherwise  it 
would  be  rapidly  discharged  by  the  ionization  currents  between 
the  upper  and  lower  atmosphere.  Our  knowledge  of  the  elec- 
trical state  of  the  upper  atmosphere  is  at  present  too  imperfect 
to  enable  us  to  determine  whether  this  distribution  of  the  charge 
is  due  to  an  effect  of  radiations  from  the  sun,  as  some  have 
supposed,  or  to  a  separation  of  the  positive  and  negative  ions 
continuously  produced  in  the  atmosphere. 

INTERNAL  HEAT  OF  THE  EARTH 

The  problem  of  the  origin  of  the  earth's  internal  heat  has 
been  a  subject  of  intermittent  discussion  for  more  than  a  cen- 
tury. The  most  plausible  and  the  generally  accepted  view  is 
that  the  earth  was  originally  a  very  hot  body,  and  in  the  course 
of  millions  of  years  has  cooled  down  to  the  present  state.  This 
process  of  cooling  is  supposed  to  be  still  continuing,  with  the 
result  that  the  earth  will  ultimately  lose  its  internal  heat  by 
radiation  into  space. 

On  this  theory,  Lord  Kelvin  bases  his  well  known  deduction 
of  the  age  of  the  earth  as  a  habitable  planet.  From  observations 

1  Simpson:  Trans.  Roy.  Soc.  Lond.  A,  p.  61,  1905. 


214  KADIOACTIVE   TRANSFORMATIONS 

of  bores  and  mines,  it  has  been  found  that  the  temperature  of 
the  earth  increases  steadily  from  the  surface  downwards,  and 
on  an  average  this  temperature  gradient  is  found  to  be  about 
1/50°  F.  per  foot,  or  .00037°  C.  per  cm.  In  order  to  obtain 
an  estimate  of  the  maximum  age  of  the  earth  on  this  theory, 
Kelvin  supposed  that  the  earth  was  initially  a  molten  mass. 
By  an  application  of  Fourier's  equation,  it  is  possible  to  de- 
duce the  temperature  gradient  at  the  surface  of  the  earth  at 
any  time  after  the  cooling  began,  provided  the  initial  tempera- 
ture and  the  average  conductivity  for  heat  of  the  materials  of 
the  earth  are  known.  Taking  the  most  probable  value  of  these 
numbers,  Kelvin  in  his  original  calculations  found  that  the  time 
required  for  the  earth  to  cool  from  the  temperature  of  a  molten 
mass  of  rock  to  its  present  state  was  about  100  million  years. 
In  later  calculations,  using  improved  data,  this  estimate  has  been 
cut  down  to  about  40  million  years. 

On  this  theory,  life  cannot  have  existed  on  the  earth  for 
more  than  40  million  years.  This  period  has  been  thought  by 
many  geologists  and  biologists  to  be  far  too  short  to  account  for 
the  processes  of  organic  and  inorganic  evolution,  and  for  the 
geologic  changes  observed  in  the  earth,  and  such  a  serious  cur- 
tailment of  the  time  at  their  disposal  has  given  rise  to  much  con- 
troversy. On  the  theory  on  which  Kelvin  bases  this  calculation, 
there  can  be  little  doubt  of  the  probable  correctness  of  this 
estimate  of  the  age  of  the  earth,  although  the  experimental  data 
on  which  the  calculations  were  based  are  of  necessity  somewhat 
imperfect.  This  theory,  however,  assumes  that  the  earth  is  a 
simple  cooling  body,  and  that  there  has  been  no  generation  of 
heat  from  internal  sources,  for  Lord  Kelvin  pointed  out  that  the 
possible  heat  developed  by  the  earth's  contraction  or  by  ordi- 
nary chemical  combination  is  not  sufficient  to  affect  appreciably 
the  general  argument. 

The  discovery  of  the  radioactive  bodies,  which  emit  during 
their  transformation  an  amount  of  heat  at  least  one  million 
times  greater  than  that  observed  in  ordinary  chemical  changes, 
throws  quite  another  light  on  this  question.  We  have  seen 
that  radioactive  matter  is  everywhere  distributed  through  the 


RADIOACTIVITY  OF  THE   EARTH  215 

surface  of  the  earth  and  in  the  atmosphere,  and  that  the  amount 
of  radium  existing  close  to  the  surface  is  of  the  order  of  several 
hundred  tons. 

It  is  of  interest  to  calculate  how  much  radium  must  be  uni- 
formly distributed  in  the  earth  in  order  to  compensate  for  the 
present  loss  of  heat  from  the  earth  by  conduction  to  the  surface. 
The  heat  in  gram  calories  per  second  lost  by  conduction  to  the 
surface  of  the  earth  is  given  by 


where  R  =  radius  of  the  earth,  K  the  heat  conductivity  of  the 
earth  in  C.  G.  S.  units,  and  T  the  temperature  gradient.  Let  X 
be  the  average  amount  of  heat  liberated  per  second  per  cubic 
centimetre  of  the  earth's  volume,  owing  to  the  presence  of 
radioactive  matter.  If  the  heat,  (?,  supplied  per  second  is  equal 
to  that  lost  by  conduction  to  the  surface,  then 

X%  TT  Es  =  4  TT  R2  K  T, 

-KT 
orX=3—  . 

Taking  the  average  value  of  K—  .004,  the  value  taken  by  Lord 
Kelvin,  and  T=  .00037,  then 

X  =  1  X  10~15  gram  calories  per  second 
=  2.2  x  10~7  gram  calories  per  year. 

Now  one  gram  of  radium  in  radioactive  equilibrium  emits 
876,000  gram  calories  of  heat  per  year.  Consequently  the  pres- 
ence of  radium  to  the  amount  of  2.6  x  10~13  grams  per  c.c.,  or 
4.6  x  10~14  grams  per  unit  mass  would  compensate  for  the  heat 
lost  by  conduction. 

In  this  calculation,  the  amount  of  radioactive  matter  present 
has  been  expressed  in  terms  of  radium.  There  is  no  doubt  that 
uranium,  thorium,  and  actinium  are  also  present,  but  the  heat- 
ing effects  of  these  are  expressed  in  terms  of  radium.  On  this 
view,  the  total  heating  effect  of  radioactive  matter  present  in  the 
•earth  is  equivalent  to  that  of  about  270  million  tons  of  radium. 


216  RADIOACTIVE   TRANSFORMATIONS 

Such  an  estimate  does  not  appear  to  be  excessive  when  it  is 
remembered  that  there  is  undoubted  evidence  that  several  hun- 
dred tons  of  radium  are  present  in  a  thin  shell  at  the  earth's 
surface.  Taking  the  estimate  of  Eve  that  about  600  tons  of 
radium  are  required  to  keep  up  the  supply  of  emanation  in  the 
atmosphere  over  the  land,  it  can  be  calculated  that  this  comes 
from  a  superficial  layer  of  the  earth,  about  18  metres  in  depth. 
This  is  based  on  the  assumption  that  the  radium  in  the  earth 
is  distributed  uniformly  in  the  amount  previously  calculated. 
Such  a  thickness  is  of  the  order  of  magnitude  to  be  expected 
from  general  considerations. 

The  experiments  of  Elster  and  Geitel  have  shown  that  radio- 
active matter  is  found  in  rocks  and  in  soils  in  about  the 
amount  required  by  this  theory.  The  heating  effect  of  this 
radioactive  matter  must  undoubtedly  be  taken  into  account  in 
deductions  based  on  the  temperature  gradient  observed  at  the 
earth's  surface.  If  the  calculated  amount  of  radium  were  dis- 
tributed uniformly  in  the  earth,  the  temperature  gradient  would 
remain  constant  as  long  as  the  supply  of  radioactive  matter 
remains  unchanged.  If  the  radioactive  matter  existed  near  the 
surface  of  the  earth  in  amounts  greater  than  this  mean  value, 
the  temperature  gradient  would  be  correspondingly  greater  than 
the  observed  value. 

While  the  data  on  which  these  deductions  are  based  is  of 
necessity  somewhat  meagre,  the  evidence  so  far  obtained  is 
sufficiently  strong  to  cast  grave  doubts  on  the  validity  of  the 
calculations  of  the  age  of  the  earth,  based  on  the  view  that  it  is 
a  simple  cooling  body.  The  temperature  gradient  observed  in 
the  earth  to-day  may  have  remained  sensibly  constant  for  mil- 
lions of  years  in  consequence  of  the  steady  generation  of  heat  in 
the  earth. 

It  does  not  seem  feasible  on  this  theory  of  the  maintenance  of 
the  earth's  heat  to  fix  with  any  certainty  the  age  of  the  earth. 
The  radium  present  in  the  earth  is  derived  from  the  parent  sub- 
stance uranium,  and  on  this  theory  uranium  must  exist  in  the 
earth  in  the  proportion  of  about  one  part  in  fifty  million.  This 
proportion  does  not  seem  excessive  from  present  data.  The 


RADIOACTIVITY   OF  THE   EARTH  217 

life  of  uranium  is  about  1000  million  years,  so  that  if  the  inter- 
nal heat  of  the  earth  were  due  entirely  to  uranium  and  radium 
the  temperature  gradient  1000  million  years  ago  would  only  be 
about  twice  that  observed  to-day. 

It  has  already  been  pointed  out  that  some  of  the  uranium 
minerals  are  undoubtedly  several  hundred  million  years  old, 
and  the  evidence  suggests  that  some  of  them  are  of  still  greater 
antiquity.  The  evidence  deduced  from  radioactive  data  alone 
points  very  strongly  to  the  conclusion  that  on  the  lowest  esti- 
mate, the  earth  is  several  hundred  million  years  old. 

The  radioactive  data  do  not  of  themselves  enable  us  to  decide 
whether  the  earth  was  originally  a  very  hot  body  or  not.  The 
theory  that  the  earth  was  originally  a  molten  mass  seems  to 
have  been  largely  the  outcome  of  an  attempt  to  explain  the 
internal  heat  of  the  earth.  Some  geologists,  notably  Professor 
Chamberlin  of  Chicago,  have  long  upheld  the  view  that  the 
geologic  evidence  by  no  means  supports  such  a  conclusion.  It 
is  not  possible  here,  however,  to  do  more  than  mention  this 
interesting  possibility. 

RADIOACTIVITY  OF  ORDINARY  MATTER 

It  is  a  matter  of  general  experience  that  every  physical  prop- 
erty discovered  for  one  element  has  been  found  to  be  shared  by 
others  in  varying  degrees.  For  example,  the  property  of  mag- 
netism is  most  marked  in  iron,  nickel,  and  cobalt,  but  every  sub- 
stance examined  has  been  found  to  be  either  feebly  magnetic  or 
diamagnetic.  It  might  thus  be  expected  on  general  principles 
that  the  property  of  radioactivity  which  is  so  marked  in  a  sub- 
stance like  radium  would  be  shown  by  other  substances. 

A  preliminary  examination  at  once  showed  that  if  ordinary 
matter  was  radioactive  at  all,  it  was  only  so  in  a  minute  degree, 
but  later  work  by  McLennan,  Strutt,  Campbell,  Wood,  and 
others,  has  shown  that  ordinary  matter  does  possess  the  prop- 
erty of  ionizing  the  gas  to  a  small  extent.  Campbell 1  in  par- 
ticular has  carefully  examined  this  question,  and  the  evidence 
obtained  by  him  affords  very  strong  proof  that  ordinary  matter 

1  Campbell:  Phil.  Mag.,  April,  1905  ;  Feb.,  1906. 


218  RADIOACTIVE   TRANSFORMATIONS 

does  possess  the  property  of  emitting  ionizing  radiations,  and 
that  each  element  emits  radiations  differing  both  in  character 
and  intensity. 

Experiments  on  this  subject  are  very  difficult,  as  the  ioniza- 
tion  currents  measured  are  extraordinarily  minute.  The  effects 
are  very  complicated  as  each  substance  emits  a  rays,  and  pene- 
trating rays,  and  the  latter  in  some  cases  give  rise  to  a  marked 
secondary  radiation. 

Campbell  concludes  that  the  a  rays  emitted  from  lead  have  a 
range  of  ionization  in  air  of  about  12.5  cms.,  while  those  from 
aluminium  have  a  range  of  only  6.5  cms.  On  an  average  the 
a  rays  emitted  from  ordinary  matter  have  a  considerably  greater 
range  in  air  than  the  a  rays  from  radium.  A  sample  of  the  lead 
employed  was  dissolved  in  nitric  acid  and  tested  by  the  ema- 
nation method  for  the  presence  of  radium,  but  not  the  slightest 
trace  was  observed. 

It  does  not  necessarily  follow  that  these  a  particles  are  iden- 
tical in  mass  with  the  a  particles  of  radium.    They  may  possibly  . 
be  hydrogen  atoms,  for  if  the  a  particles  from  ordinary  matter 
were  helium  atoms  we  should  expect,  for  example,  to  find  helium 
in  lead. 

If  the  expulsion  of  a  particles  be  taken  as  evidence  of  atomic 
disintegration,  a  simple  calculation  shows  that  the  life  of  ordi- 
nary matter  is  of  the  order  of  at  least  one  thousand  times  that 
of  uranium,  i.  e.  not  less  than  1012  years. 


CHAPTER   X 
PROPERTIES  OF  THE  a  RAYS 

IN  the  previous  chapters  we  have  shown  how  prominent 
a  role  the  a  rays  play  in  radioactive  phenomena,  as  compared 
with  the  more  penetrating  /3  and  7  rays.  Not  only  are  they 
responsible  for  most  of  the  ionization  observed  in  the  neighbor- 
hood of  radioactive  matter,  but  they  are  also  directly  concerned 
with  the  rapid  emission  of  heat  energy  from  these  substances ; 
in  addition,  they  generally  accompany  the  transformation  of  the 
different  types  of  radioactive  matter,  while  the  /3  and  7  rays  are 
emitted  only  in  the  case  of  -a  few  products.  Finally,  we  have 
seen  that  there  is  good  reason  to  believe  that  the  a  particle  is  to 
be  identified  with  an  atom  of  helium. 

In  this  chapter,  we  shall  outline  in  some  detail  the  more  im- 
portant properties  possessed  by  the  a  rays,  and  especially  by  the 
a  rays  emitted  by  radium  and  its  products.  On  account  of 
their  great  intensity,  the  a  rays  frQm  radium  have  been  more 
easily  studied  than  the  corresponding  rays  from  feebly  radioac- 
tive substances  like  uranium  and  thorium.  At  the  same  time, 
the  evidence  so  far  obtained  indicates  that  the  a  particles  from 
all  the  radioactive  substances  have  the  same  mass,  and  differ  for 
each  product  only  in  their  initial  velocity  of  projection. 

The  a  rays  differ  from  the  /3  and  7  rays  in  the  ease  with 
which  they  are  absorbed  by  matter  and  by  the  comparatively 
large  ionization  they  produce  in  the  air  near  to  a  radioactive 
body.  By  examining  the  effect  of  adding  thin  screens  of  metal- 
lic foil  over  radioactive  matter,  it  was  found  that  the  a  rays 
from  the  radioactive  substances  differed  in  penetrating  power. 

We  shall  see  later  that  the  a  rays  from  radium  are  completely 
cut  off  by  a  layer  of  aluminium  foil  of  thickness  .04  mms.,  or  by 
a  layer  of  air  of  thickness  7  cms.  The  ionizing  action  of  the  a 
rays  is  consequently  confined  within  a  short  distance,  while  that 


220  RADIOACTIVE   TRANSFORMATIONS 

of  the  /3  rays  extends  for  several  metres,  and  that  of  the  7  rays 
for  several  hundred  metres. 

The  a  rays  were  at  first  thought  to  be  non-deflectable  by  a 
magnetic  field,  for  the  application  of  a  magnetic  field  sufficiently 
strong  to  bend  away  completely  the  (3  rays  had  no  appreciable 
effect  on  the  a  rays. 

In  1901,  the  writer  began  experiments  by  the  electric  method, 
to  see  if  the  a  rays  could  be  deflected  in  a  strong  magnetic  field, 
but  with  the  weak  preparations  of  radium  (activity  1000)  then 
available,  the  electric  effects  were  too  small  to  push  the  experi- 
ments to  the  necessary  limit.  Later,  in  1902,  using  a  prepara- 
tion of  activity  19,000,  the  experiments  l  were  successful,  and 
the  a  rays  were  found  to  be  deflected  in  passing  through  both  a 
magnetic  and  an  electric  field. 

The  direction  of  the  deflection  was  opposite  to  that  for  the  @ 
rays,  and  this  indicated  that  the  a  rays  consisted  of  a  flight  of 
positively  charged  particles.  By  measurement  of  the  amount  of  / 
deflection  of  the  rays  in  passing  through  magnetic  and  electric 
fields  of  known  strengths,  the  mass  and  velocity  of  the  a  par- 
ticle were  determined.  The  value  of  e/m  —  the  ratio  of  the 
charge  carried  by  an  a  particle  to  its  mass  —  was  found  to  be 
about  6  X  103,  while  the  maximum  velocity  v  of  the  particle 
was  found  to  be  2.5  x  109  cms.  per  second. 

Since  the  ratio  e/m  for  the  hydrogen  atom  is  about  104,  this 
result  indicated  that  the  a  particle  was  atomic  in  size,  and, 
assuming  that  the  a  particle  carried  the  same  charge  as  a  hydro- 
gen atom,  had  a  mass  about  twice  that  of  the  hydrogen  atom. 
It  must  be  remembered  that  the  amount  of  the  deviation  of  the 
rays  in  a  given  magnetic  field  is  minute  in  comparison  with  that 
of  the  /3  rays.  For  example,  the  swiftest  a  particle  projected 
from  radium  at  right  angles  to  a  magnetic  field  of  10,000  C.  G.  S. 
units  describes  the  arc  of  a  circle  of  40  cms.  radius.  The  swifts 
est  0  particle  from  radium,  which  is  projected  with  96  per  cent 
of  the  velocity  of  light,  describes  under  similar  conditions  a 
circle  of  about  5  mms.  radius. 

i  Rutherford  :  Physik.  Zeit.,  iv,  p.  235  (1902) ;  Phil.  Mag.,  Feb.,  1903. 


PRpPERTIES    OF   THE   a   RAYS  221 

Becquerel 1  confirmed  the  magnetic  deflection  of  the  a  rays 
from  radium  by  means  of  the  photographic  method,  and  also  L 
showed  that  the  a  rays  from  polonium  have  a  similar  property. 
Using  some  pure  radium  bromide  as  a  source  of  rays,  Des  Cou- 
dres  a  measured  the  deflection  of  a  pencil  of  a  rays  in  a  vacuum 
after  passing  through  a  magnetic  and  electric  field.  He  found 
e/m  to  be  6.3  x  103  and  the  velocity  to  be  1.64  x  109  cms.  per 
second.  The  values  of  e/m  obtained  by  Rutherford  and  Des 
Coudres  were  in  good  agreement,  but  the  velocities  varied  con- 
siderably. In  the  experiments  of  Des  Coudres  the  a  rays  were 
passed  through  a  screen  of  aluminium.  It  will  be  seen  later 
that  this  reduces  the  velocity  of  the  a  particles,  and  that  the 
correct  velocity  of  the  swiftest  a  particle  from  radium  is  about 
2  x  109  cms.  per  second,  or  about  1/15  of  the  velocity  of  light. 

In  1905,  the  question  was  again  attacked  by  Mackenzie,3 
using  pure  radium  bromide  as  a  source  of  rays.  A  photographic 
method  was  employed,  in  which  the  a  rays  fell  on  a  glass  plate 
coated  on  its  lower  surface  with  zinc  sulphide.  A  photographic 
plate  was  placed  on  the  upper  side  of  the  glass  plate,  and  was 
acted  on  by  the  light  from  the  scintillations  produced  by  the  a 
rays  in  the  zinc  sulphide  screen  immediately  below  it.  The 
deflection  of  the  pencil  of  rays  was  observed  as  before  after  pass- 
ing through  a  magnetic  and  electric  field.  The  a  rays  were  „ 
found  to  be  unequally  deflected  by  a  magnetic  field,  showing 
that  the  a  particles  varied  either  in  mass  or  velocity.  This  dis- 
persion of  the  rays  by  a  magnetic  and  electric  field  made  it  diffi- 
cult to  deduce  the  constants  of  the  rays  with  accuracy.  Taking 
the  mean  value  for  the  dispersion  of  the  deflected  pencils,  he 
found  the  value  of  e/m  to  be  4.6  x  103,  and  the  velocity  of  the  a 
particles  to  vary  between  1.3  x  109  and  1.96  x  109  cms.  per  sec- 
ond, on  the  assumption  that  the  a  particles  all  carry  the  same 
charge  and  have  the  same  mass. 

The   importance   of  an   accurate  determination   of    efm   for 
the  a  particle  had  long  been  recognized  because  of  the  light  it 

1  Becquerel:  Comptes  rendus,  cxxxvi,  pp.  199,  431  (1903). 

2  Des  Coudres:  Physik.  Zeit.,  iv,  p.  483  (1903). 

3  Mackenzie:  Phil.  Mag.,  Nov.,  1905. 


222  RADIOACTIVE   TRANSFORMATIONS 

would  throw  on  the  question  whether  the  a  particle  is  an  atom 
of  helium.  In  all  the  methods  so  far  described,  a  thick  layer  of 
radium  in  radioactive  equilibrium  was  employed  as  a  source  of 
rays.  On  the  theory  of  absorption  of  the  a  rays,  put  forward  by 
Bragg  and  Kleeman,  which  will  be  discussed  later,  it  was  recog- 
nized that  the  a  rays  emitted  from  a  thick  or  a  thin  layer  of 
radium  must  consist  of  a  particles  moving  at  different  speeds. 
The  use  of  a  complex  pencil  of  rays  was  open  to  very  serious 
objections,  for  it  was  impossible  to  know  whether  the  rays  most 
deflected  in  a  magnetic  field  corresponded  to  the  most  deflected 
rays  in  an  electric  field  or  not. 

The  simplest  method  of  accurately  determining  the  value  of 
e/m  is  to  use  a  homogeneous  source  of  rays,  i.  e.,  to  use  a  radio- 
active substance  in  which  all  the  a  particles  escape  at  the  same 
speed.  The  writer  found  that  a  wire  made  active  by  exposure 
to  the  radium  emanation  completely  satisfied  these  essentials. 
The  active  matter,  consisting  of  radium  A,  B,  and  C,  is  deposited 
in  an  extremely  thin  film  on  a  negatively  charged  wire  exposed 
in  the  presence  of  the  emanation.  After  three  hours'  exposure, 
the  activity  of  the  wire  reaches  a  maximum  value.  After 
removal,  radium  A,  which  has  a  three  minute  period,  is  rapidly 
transformed,  and  has  practically  disappeared  after  fifteen  min- 
utes. The  activity  remaining  is  then  entirely  due  to  radium  C. 
The  a  particles  from  radium  C  are  all  expelled  with  identically 
the  same  velocity,  for  there  is  no  appreciable  dispersion  of  the 
rays  in  a  magnetic  field.  The  particles  projected  into  the  wire 
are  completed  absorbed,  and  those  which  escape  do  not  suffer  in 
velocity  in  their  passage  through  the  very  thin  film  of  intensely 
active  matter. 

Using  10  to  20  milligrams  of  radium  in  solution,  a  wire  one 
centimetre  long  can  be  made  extremely  active  by  an  arrange- 
ment similar  to  that  shown  in  Fig.  24,  page  100.  The  wire  pro- 
duces a  strong  photographic  impression  on  a  plate  brought 
near  it.  The  chief  drawback  to  such  a  source  of  rays%  is  that 
the  intensity  of  the  rays  falls  off  rapidly,  and  two  hours  after 
removal  is  only  14  per  cent  of  the  initial  value. 

The  apparatus  shown  in  Fig.  42  is  very  convenient  for  the 


PROPERTIES   OF   THE   a  RAYS 


223 


determination  of  the  magnetic  deflection  of  the  rays.  An  active 
wire  is  placed  in  a  groove  A.  The  rays  pass  through  a  narrow 
slit  B  and  fall  on  a  small  piece  of  photographic  plate  at  C.  The 
apparatus  is  enclosed  in  a  cylindrical  vessel  P,  which  can  be 
rapidly  exhausted  of  air.  The  apparatus  is  placed  between  the 
pole  pieces  of  a  large  electromagnet,  so  that  the  magnetic  field 
is  parallel  to  the  direction  of  the  wire  and  slit,  and  uniform 
over  the  whole  path  of  the  rays.  The  electromagnet  is  excited 
by  a  constant  current,  which  is  reversed 
every  ten  minutes.  On  developing  the 
plate  two  well  defined  bands  are  observed 
corresponding  to  the  pencils  of  rays  which 
have  been  deflected  equally  on  opposite 
sides  of  the  normal. 

If  p  is  the  radius  of  curvature  of  the 
circle  described  by  the  rays  in  a  uniform 

field  of  strength  H,  then  Hp  =  — ,  where 


v  is  the  velocity  of  the  rays,  e  the  charge 
on  the  particle,  and  m  its  mass. 

Let  d  =  deflection  of  the  rays  from 
the  normal  measured  on  the  photographic 
plate, 

a  =  distance  of  plate  from  slit, 

b  =  distance  of  slit  from  the  source. 


FIG.  42. 

Apparatus  for  deter- 
mining the  amount  of  de- 
flection of  a  pencil  of  a 
rays  in  a  strong  magnetic 
field. 


Then  by  a  property  of  the  circle,  if  the  deflection  d  is  small 

compared  with  a, 

2pd  =  a  (a  +  b). 
Consequently, 

mv  Ha  (a  -f  b) 

—  =     =  -  -  — 


In  the  actual  photographs,  using  the  wire  as  a  source  of  rays, 
the  traces  of  the  pencil  of  rays  stand  out  clearly  with  well 
defined  edges,  so  that  the  value  2d,  the  distance  of  the  inside 
edge  of  one  band  to  the  outside  edge  of  the  other,  can  easily 
be  measured. 


224  RADIOACTIVE   TRANSFORMATIONS 

The  value  of  Hp  for  the  a  particles  emitted  from  radium  C 
was  found  in  this  way  to  be  4.06  x  105.  In  a  field  of  10,000 
C.  G.  S.  units,  the  a  particle  consequently  describes  a  circle  of 
radius  40.6  cms. 

RETARDATION  OF  THE  VELOCITY  OF  THE  a  PARTICLE  IN 

PASSING  THROUGH   MATTER 

It  was  found  by  the  writer l  that  the  velocity  of  the  a  par- 
ticle diminishes  in  its  passage  through  matter.  This  is  most 
simply  shown  by  a  slight  modification  of  the 
experimental  arrangement,  described  above,  which 
has  been  used  by  Becquerel.  By  means  of  mica 
plates,  placed  at  right  angles  to  the  slit,  the  appa- 
ratus is  divided  into  two  equal  parts.  One  half 
of  the  photographic  plate  is  acted  on  by  the  rays 
from  the  bare  wire,  and  the  other  by  the  rays 
which  have  passed  through  an  absorbing  screen 
placed  over  the  wire. 

A  drawing  of  a  photograph  obtained  by  this 
method  is  shown  in   Fig.   43.     The   two  upper 
FIG.  43.          bands  A  represent  the  traces  of  the  pencil  of  rays 
Retardation  of   obtained  by  reversal  of  the  magnetic  field  for  the 
the    velocity    of  fn)m    the    uncovered  half  of  the  wire  .   the 

the  a  particles  in        J  .  ' 

passing  through  lower  bands  B  were  obtained  tor  the  rays  from 
matter.  the  wire,  when  covered  with  eight  layers  of  alu- 

minium foil,  each  of  thickness  about  .00031  cms. 
The  apparatus  was  exhausted  during  the  experiment,  so  that 
the  absorption  of  the  rays  in  air  is  negligible. 

The  greater  deflection  of  the  pencil  of  rays  which  have  passed 
through  the  aluminium  is  clearly  seen  in  Fig.  43.  It  will  be 
shown  later  that  the  value  of  e/m  for  the  particles  does  not  ' 
change  in  consequence  of  their  passage  through  matter.  The 
greater  deflection  of  the  rays  is  then  due  to  a  decrease  of  their 
velocity  after  passing  through  the  screen.  This  velocity  is 
inversely  proportional  to  the  distance  between  the  centres  of  the 
bands. 

1  Rutherford  :  Phil.  Mag.,  July,  1905  ;  Jan.  and  April,  1906. 


PROPERTIES   OF  THE   a  RAYS 


225 


We  have  seen  that  the  a  particles  from  radium  C  are  all  pro- 
jected initially  at  the  same  speed.  The  absence  of  dispersion 
of  the  rays  after  passing  through  the  screen  shows  that  the 
velocity  of  all  the  a  particles  is  reduced  by  the  same  amount  in 
traversing  the  screen. 

The  following  table  gives  the  velocity  of  the  a  particles  from 
radium  C  after  passing  through  successive  layers  of  aluminium 
foil,  each  of  thickness  about  .00030  cms.  The  velocity  is  ex- 
pressed in  terms  of  F"0,  the  velocity  of  the  a  particles  from 
radium  C  with  an  uncovered  wire. 


Number  of  layers  of 
aluminium  foil. 

Velocity  of 
a  particles. 

0 

1.00  F0 

2 

.94    ' 

4 

.87    < 

6 

.80    ' 

8 

.72    ' 

10 

.63    ' 

12 

.53    ' 

14 

.43  " 

14.5 

not  measurable 

There  is  a  marked  weakening  of  the  photographic  effect  of  the 
rays  after  passing  through  10  layers  of  foil.  The  photographic 
impression  is  weak,  but  distinct  with  13  layers,  and,  using  very 
active  wires,  can  be  observed  with  14  layers.  On  account  of 
this  falling  off  of  the  photographic  effect  of  the  rays,  very  active 
wires  are  required  to  produce  an  appreciable  darkening  of  the 
plate  through  more  than  12  layers  of  foil.  The  lowest  velocity 
of  the  a  particle  so  far  observed  was  about  .4  FJ>,  which  cor- 
responded to  the  velocity  of  the  rays  after  passing  through  14 
layers  of  foil.  The  photographic  action  of  the  a  rays  steadily 
diminishes  with  increase  of  the  absorbing  layer,  but  falls  off 
very  rapidly  for  a  thickness  greater  than  10  layers  of  aluminium. 
The  velocity  of  the  a  particle  measured  in  this  way  is  still 
considerable  when  the  photographic  action  has  almost  ceased. 
Such  a  result  suggests  that  there  is  a  critical  velocity  of  the  a 
particles,  below  which  they  are  unable  to  affect  appreciably  a 

photographic  plate. 

15 


226  RADIOACTIVE   TRANSFORMATIONS 

A  similar  abrupt  falling  off  is  observed  in  the  ionizing  and 
phosphorescent  effects  of  the  rays.  From  observations  on  thin 
layers  of  radium,  Bragg  found  that  the  ionizing  action  of  the 
rays  from  radium  C  ceased  comparatively  abruptly  after  travers- 
ing 7.06  cms.  of  air.  A  similar  result  was  observed  later  by 
McClung  by  using  an  active  wire  coated  with  radium  C  as  a 
source  of  rays. 

In  a  similar  way  the  writer  found  that  the  scintillations  pro- 
duced by  the  a  rays  on  a  screen  of  zinc  sulphide  ceased  sud- 
denly when  the  rays  passed  through  6.8  cms.  of  air.  If  layers 
of  aluminium  foil  are  placed  over  the  active  wire,  the  range 
of  ionization  and  phosphorescence  is  diminished  by  a  definite 
amount  for  each  layer.  Each  layer  of  foil  of  the  thickness 
used  in  the  photographic  experiment  was  equivalent  in  stopping 
power  to  about .  50  cms.  of  air.  A  photographic^effect  of  the  a 
rays  was  just  observable  through  14  layers  of  aluminium.  This 
corresponds  to  7.0  cms.  of  air,  —  nearly  the  same  range  at  which 
the  ionizing  and  phosphorescent  effects  vanish.  The  three  char- 
acteristic actions  of  the  a  rays  thus  cease  together  when  the 
rays  have  passed  through  a  definite  distance  of  air  or  a  definite 
thickness  of  an  absorbing  screen.  Unless  the  velocity  of  the 
a  particle  falls  off  with  great  rapidity  at  the  end  of  its  course 
in  air,  it  would  appear  as  if  there  were  a  critical  velocity:  of 
/the  a  particle  below  which  it  produced  no  appreciable  ioniz- 
ing, photographic,  or  scintillating  effect.  This  property  of  the 
a  particles  will  be  discussed  in  more  detail  later.  In  any 
case  the  rapid  falling  off  of  these  three  actions  of  the  a  par- 
ticle at  the  end  of  its  range  indicates  that  there  is  a  close  con- 
nection between  them.  The  photographic  action  of  the  a  particle 
falls  off  in  the  same  rapid  manner  as  the  ionizing  action,  and  it 
seems  reasonable  to  suppose  that  the  effect  of  the  rays  on  a  pho- 
tographic plate  is  the  result  of  the  ionization  of  the  silver  salts. 

In  a  similar  way,  it  is  possible  that  the  scintillations  observed 
in  zinc  sulphide  are  primarily  caused  by  ionization  of  the  sub- 
stance, and  that  the  scintillations  may  arise  as  a  result  of  the 
recombination  of  such  ions.  The  brightness  of  the  scintillations 
certainly  depends  on  the  velocity  of  the  a  particle.  If  the  effect 


PROPERTIES    OF   THE   a  RAYS 


227 


of  the  a  rays  on  zinc  sulphide  is,  as  some  have  supposed,  purely 
mechanical,  and  the  scintillations  result  from  a  cleavage  of  the 
crystals,  it  is  not  easy  to  see  why  this  effect  should  fall  off 
suddenly,  although  the  energy  possessed  by  the  particle  is  still 
considerable. 

ELECTROSTATIC  DEFLECTION  OF  THE  a  RAYS 

In  order  to  measure  the  deflection  of  the  a  rays  from  radium 
C  in  an  electric  field,  the  arrangement  shown  in  Fig.  44  was 
adopted. 

The  rays  from  the  active 
wire  W,  after  traversing  a 
thin  mica  plate  in  the  base  of 
the  brass  vessel  M,  passed  be- 
tween two  parallel  insulated 
plates  A  and  B,  about  4  cms. 
high  and  0.21  mm.  apart. 
The  distance  between  the 
plates  was  fixed  by  thin 
strips  of  mica  placed  be- 
tween them.  The  terminals 
of  a  storage  battery  were  con- 
nected with  A  and  B,  so  that 
a  strong  electric  field  could 
be  produced  between  the 
two  plates.  The  pencil  of 
rays  after  emerging  from  the 
plates  fell  on  a  photographic 
plate  P  placed  a  definite  dis- 
tance above  the  plates.  By 
means  of  a  mercury  pump 

the  vessel  was  exhausted  to  a  low  vacuum.  In  their  passage 
between  the  charged  plates,  the  a  particles  describe  a  parabolic 
path,  and  after  emergence  travel  in  a  straight  line  to  the  photo- 
graphic plate.  By  reversing  the  electric  field  the  deflection  of 
the  pencil  of  rays  is  reversed. 

In  Fig.  45  A  shows  the  natural  width  of  the  line  on  the  plates 


','.   I'P 

• 

\\  u 

1.1  If 

ll    If 

II    if 

1 

i 
,'< 

'! 

I 

I 
i 

M 

/? 

m 

B 

I§J 

___TT_ 

FIG.  44. 

Apparatus  for  determining  the  deflection 
of  a  rays  in  passing  through  a  strong 
electric  field. 


228 


RADIOACTIVE   TRANSFORMATIONS 


when  no  electric  field  is  acting ;  B  and  C  show  the  traces  of 
the  deflected  pencils  of  rays  for  potential  differences  between  the 
plates  of  340  and  497  volts  respectively.  For  small  voltages  the 
natural  width  of  the  line  is  broadened ;  for  increased  voltages 
the  single  line  breaks  into  two  and  the  width  of  the  lines  steadily 
narrows.  Such  an  effect  is  to  be  expected  theoretically.  It  can 
be  easily  shown  that  if  D  is  the  distance  between  the  extreme 
edges  of  the  deflected  band  for  a  potential  difference  E, 


mv2 


SEl* 


(B-df 


where  e  is  the  charge  on 
the  a  particle,  m  its  mass, 
v  its  velocity,  I  the  dis- 
tance of  the  photographic 
plate  from  the  end  of  the 
parallel  plates,  d  the  dis- 
tance between  the  parallel 
plates.  This  simple  equa- 
tion holds  only  if  the  field 
is  strong  enough  to  deflect 
the  a  particle  through  a 
distance  greater  than  d  in 

its  passage  through  the  electric  field.     For  small  values  of  the 
field,  a  modified  form  of  this  equation  must  be  used. 

The  decrease  in  velocity  of  the  rays  in  passing  through  the 
mica  screen  was  separately  determined.  In  most  of  the  experi- 
ments, the  mica  plate  reduced  the  velocity  of  the  a  particles 
from  radium  C  by  24  per  cent. 

WYI     W 

From  the    magnetic   deflection  the  value    of  -   -  is  known,, 


FIG.  45. 

Electrostatic  deflection  of  the  a  rays.  The 
bands  are  drawn  to  scale  from  the  actual  pho- 
tographs. Magnification  about  3  times. 


„  m  v*  . 
of  is 


while  from   the   electrostatic  deflection,  the   value 
determined. 

From  these  two  equations  the  values  of  e/m  and  v  can  at  once 
be  deduced.     Examined  in  this  way,  it  was  found  that : 1  — 


1  Rutherford  :  Phvs.  Review,  Feb.,  1906. 


PROPERTIES    OF   THE   a   RAYS  229 

(1)  The  value  of  e/m  was  unaltered  by  the  passage  of  the  a     „ 
particles  through  matter ; 

(2)  The  value  of  e/m  was  very  nearly  5  X  103; 

(3)  The  initial  velocity  of  projection  of  the  a  particles  from 
radium  C  was  2  x  109  cms.  per  second. 

A  similar  method  was  applied  to  determine  the  value  of  e/m 
and  the  velocity  of  the  a  particles  emitted  from  radium  A  and 
radium  F  (radiotellurium).  In  both  cases  the  value  of  e/m 
was  5  x  103  within  the  limits  of  experimental  error.  The  initial 
velocity  of  the  a  particles  from  radium  A  was  about  86  per  cent  V 
of  that  of  the  a  particle  from  radium  C,  while  the  velocity  of  * 
the  a  particle  from  radiotellurium  was  about  80  per  cent  of  that 
of  the  a  particle  from  radium  C.  The  experiments  on  the  ve- 
locity and  value  of  e/m  for  the  a  particles  from  radium  itself 
and  from  the  emanation  are  not  yet  fully  completed,  but  the 
results  so  far  obtained  indicate  that  the  value  of  e/m  will  be  the 
same  as  in  the  other  cases. 

Such  results  show  conclusively  that  the  a  particles  from 
radium  and  its  products  have  identical  mass,  but  differ  in  the 
initial  velocities  of  their  projection.  The  arguments  in  favor  of 
the  view  that  the  a  particle  consists  of  an  atom  of  helium  carry- 
ing two  ionic  charges  have  already  been  discussed  in  detail  on 
pagelL84) 

Dr.  Hahn,  working  in  the  laboratory  of  the  writer,  has  found 
that  the  a  rays  emitted  by  thorium  B  are  deflected  both  in  a  mag-  " 
netic  and  in  an  electric  field.  These  rays  have  a  velocity  about 
10  per  cent  greater  than  those  from  radium  C,  but  have  the 
same  value  of  e/m.  In  these  experiments,  the  thorium  B  was 
deposited  on  a  thin  negatively  charged  wire,  by  exposure  to  the 
thorium  emanation  emitted  by  the  very  active  preparation  of 
radio  thorium  separated  by  Hahn  (see  page  68).  It  was  also 
found  that  the  range  in  air  of  the  a  particles  expelled  from 
thorium  B,  determined  both  by  the  electrical  and  scintillation 
methods,  was  about  8.6  cms.,  or  about  1.6  cms.  greater  than  that 
for  the  a  particles  from  radium  C. 

Since  the  mass  of  the  a  particle  from  thorium  B  and  the 
radium  products  is  the  same,  it  appears  probable  that  the  same 


230  RADIOACTIVE   TRANSFORMATIONS 

equality  also  holds  for  the  a  particles  from  the  other  thorium 
products.  The  mass  of  the  a  particles  from  actinium  has  not 
yet  been  measured,  but  there  is  every  reason  to  believe  that  it 
has  the  same  value  as  for  radium.  On  this  view,  the  only  com- 
mon  product  of  the  different  radioactive  bodies  is  the  a  particle, 
which,  as  we  have  seen,  is  a  projected  helium  atom. 

SCATTERING  OF  THE  a  RAYS 

It  is  well  known  that  a  narrow  incident  beam  of  ft  or  cathode 
rays  is  scattered  in  its  passage  through  matter,  so  that  the  emer- 
gent pencil  of  rays  is  no  longer  well  defined.  This  scattering  of 
the  ft  rays  increases  as  the  velocity  of  the  ft  particles  diminishes. 
In  a  theoretical  paper,  Bragg l  pointed  out  that  this  scattering 
of  the  ft  rays  is  to  be  expected.  The  ft  particle  in  its  passage 
through  the  molecules  of  matter  enters  the  electric  field  of  the 
atom,  and  its  direction  of  motion  is  consequently  changed.  The 
smaller  the  kinetic  energy  of  the  ft  particle  the  greater  will  be 
the  deflection  of  the  path  of  some  of  the  rays.  If  a  narrow  pen- 
cil of  ft  rays  falls  on  an  absorbing  screen,  a  portion  of  the  rays 
will  suffer  so  much  deflection  that  the  emerging  beam  will  con- 
sist of  a  much  wider  cone  of  rays. 

On  account  of  their  much  greater  kinetic  energy,  it  is  theV 
oretically  to  be  expected  that  the  a  particles  will  suffer  much 
less  deflection  of  their  path  in  their  passage  through  matter  than 
the  ft  rays.  The  a  particles  must  move  nearly  in  a  straight  v 
line,  and  pass  directly  through  the  atoms  or  molecules  of  matter 
in  their  path,  without  much  change  in  their  direction  of  motion. 
This  theoretical  conclusion  of  Bragg  is  borne  out  by  experi- 
ment. The  scattering  of  the  a  particles  is  very  small  compared 
with  that  of  the  ft  particles  moving  at  the  same  speed,  so  that  a 
narrow  pencil  of  a  rays  after  traversing  an  absorbing  screen  is 
still  well  defined  after  emergence.  At  the  same  time  there  is 
undoubtedly  a  small  scattering  of  the  rays  in  their  passage 
through  matter,  which  must  be  taken  into  account. 

If  the  a  rays  pass  through  air,  for  example,  the  width  of  the 
trace  of  a  pencil  of  a  rays  on  a  photographic  plate  is  always 
1  Bragg  :  Phil.  Mag.,  Dec.,  1904. 


PROPERTIES    OF   THE   a  RAYS  231 

broader  than  in  a  vacuum.  In  addition  the  edges  of  the  bands 
are  not  nearly  so  well  denned  in  air  as  in  a  vacuum.  This  result 
shows  that  some  of  the  a  particles  have  suffered  a  change  of  di- 
rection of  motion  in  their  passage  through  the  molecules  of  air. 

The  arrangement  adopted  to  determine  the  retardation  of  the 
velocity  of  the  a  particles  in  their  passage  through  matter 
(Fig.  43)  is  not  complicated  by  the  scattering  of  the  rays,  since 
the  absorbing  screen  is  placed  over  the  active  wire  between  the 
source  and  the  slit.  If,  however,  the  absorbing  screen  is  placed 
over  the  slit,  the  scattering  of  the  a  particles  is  at  once  seen  by 
the  broadening  of  the  trace  of  the  rays  on  the  plate.  A  broad 
diffuse  impression  is  observed  on  the  plate  instead  of  the  narrow 
band  with  well  defined  edges,  observed  when  the  absorbing 
screen  is  placed  below  the  slit.  The  amount  of  scattering 
increases  with  the  thickness  of  the  screen.  When  eleven  layers 
of  aluminium  foil  were  placed  over  the  slit,  —  an  amount  nearly 
sufficient  to  cut  off  the  ionizing  and  photographic  effects  of  the 
rays  —  an  examination  of  the  photographic  plate  showed  that 
some  of  the  rays  had  been  deflected  about  3°  from  the  normal. 
A  part  of  the  rays  may  have  been  deflected  through  a  consider- 
ably greater  angle,  but  their  photographic  action  was  too  small 
to  be  detected. 

We  may  thus  conclude  that  the  path  of  the  a  particles,  espe- 
cially when  their  velocity  is  reduced,  is  deflected  to  an  appre- 
ciable extent  in  passing  through  matter.  The  fact  that  the 
direction  of  motion  of  an  a  particle  possessing  such  great  energy 
of  motion,  can  be  changed  in  its  passage  through  matter,  shows 
that  there  must  exist  a  very  strong  electric  field  within  the  atom, 
or  in  its  immediate  neighborhood.  The  change  of  direction  of 
3°  in  the  direction  of  motion  of  the  particles  in  passing  through 
a  distance  of  .003  cms.  of  matter  would  require  an  average  trans- 
verse electric  field  over  this  distance  corresponding  to  more  than 
20  million  volts  per  centimetre.  Such  a  result  shows  out  clearly 
that  the  atom  must  be  the  seat  of  very  intense  electrical  forces 
—  a  deduction  in  harmony  with  the  electronic  theory  of  matter. 

We  have  seen  that  the  velocity  of  the  a  particles  from  radium 
C  lose  their  photographic  action  when  their  velocity  falls  to 


232  RADIOACTIVE   TRANSFORMATIONS 

J  about  40  per  cent  of  the  initial  value.  On  account  of  the  com- 
plications introduced  by  the  scattering  of  the  a  particles  in 
passing  through  matter,  it  is  difficult  to  decide  with  certainty 
whether  this  " critical  velocity"  of  the  a  particle,  below  which 
it  fails  to  produce  its  characteristic  effects,  is  a  real  or  only  an 
apparent  property  of  the  rays.  Without  discussing  the  evidence 
in  detail,  I  think  there  is  undoubted  proof  that  this  critical 
velocity  of  the  a  particle  has  a  real  existence. 

PHOTOGRAPHIC  EFFECTS  OF  THE  a  RAYS  FKOM  A  THICK 
LAYER  OF  RADIUM 

Since  the  a  particles  emitted  by  radium  and  its  products 
decrease  in  velocity  in  their  passage  through  matter,  the  radia- 
tions emerging  from  a  thick  layer  must  consist  of  particles 
moving  at  widely  different  speeds.  This  must  obviously  be  the 
case,  since  the  a  particles  which  come  from  some  depth  below 
the  radiating  surface  are  retarded  in  their  passage  through  the 
radium  itself. 

A  pencil  of  rays  from  radium  is  consequently  complex,  and  if 
a  magnetic  field  is  applied  perpendicularly  to  the  direction  of  the 
rays,  each  particle  will  describe  the  arc  of  a  circle,  the  radius  of 
which  is  directly  proportional  to  the  velocity  of  the  particle. 

This  unequal  deflection  of  the  a  particles  in  a  magnetic  field 

ogives   rise   to  a  "magnetic^  spectrum,"  in  which  the   natural 

width  of  the  trace  is  much  increased.     This  dispersion  of  the 

complex  pencil  of  rays  has  been  observed  by  Mackenzie1  and 

by  the  writer.2 

We  have  seen  that  the  a  particle  has  comparatively  little 
photographic  effect  when  its  velocity  falls  to  about  .6  V0,  where 
V0  is  the  maximum  velocity  of  the  a  particle  from  radium  C. 
Since  the  a  particles  from  the  latter  have  a  greater  speed  than 
the  a  particles  from  any  other  radium  product,  we  might  thus 
expect  to  obtain  a  magnetic  spectrum  corresponding  to  a  par- 
ticles whose  velocity  lies  between  .6  V0  and  V0.  In  an  actual 
photograph  of  a  deflected  pencil  of  rays,  the  writer  observed  the 

1  Mackenzie  :  Phil.  Mag.,  Nov.,  1905. 

2  Rutherford:  Phil.  Mag.,  Jan.,  1906. 


PROPERTIES    OF    THE    a   RAYS 


233 


presence  of  rays  whose  velocities  lay  between  .67  V0  and  .95  V0 ; 
while  Mackenzie,  by  the  method  of  scintillations,  observed  the 
presence  of  rays  having  velocities  between  .65  V0  and  .98  V0. 

When  we  remember  that  the  photographic  action  of  the  /3  and 
7  rays  from  the  radium  prevents  the  detection  of  weak  photo- 
graphic effects  produced  by  the  a  rays,  the  observations  are  seen 
to  be  in  good  agreement  with  theory. 

Becquerel1  early   observed  an  interesting  peculiarity  in  the 
deflection  of  a  pencil  of  a  rays  from  a  thick  layer  of  radium  in 
passing  through  a  uniform  magnetic  field.     A  narrow  vertical 
pencil  of  rays  fell  on  a  photographic  plate  which  was  placed 
at  right  angles  to  the  slit  and  inclined  at  a  small 
angle  with  the  vertical.     By  reversing  the  mag- 
netic field,  two  fine  diverging  lines  S  P,  S  P', 
were  observed  on  the  plate  (see  Fig.  46).     The 
distance  between  these  lines  at  any  point  repre- 
sents twice  the  deflection  of  the  pencil  of  rays 
from  the  normal  at  that  point.  By  careful  meas- 
urement, Becquerel  found  that  these  two  diverg- 
ing lines  were  not  accurately  the  arcs  of  a  circle, 
but  that  the  radius  of  curvature  of  the  path  of 
the  rays  increased  with  the  distance  from  the 
source.     Becquerel  thought  that  the  a  rays  from 
radium  were  homogeneous,  and  concluded  from 
this  experiment  that  the  value  of  e/m  of  the 
particles  progressively  decreased  in  their  passage 
through  air,  in  consequence  of  an  increase  of  m  by  accretions 
from  the  air. 

Bragg,2  however,  showed  that  this  peculiarity  in  the  trace  of 
the  rays  could  be  simply  explained  without  any  assumption  of 
an  alteration  in  the  value  of  e/m,,  by  taking  into  account  the  com- 
plexity of  the  pencil  of  rays.  The  experimental  arrangement 
is  diagrammatically  shown  in  Fig.  46.  S  P  and  S  P'  represent 
the  diverging  traces  of  the  rays  on  the  photographic  plate  in  a 
uniform  magnetic  field  after  emerging  from  the  slit  S.  Let  us 

1  Becquerel:  Comptes  rendus,  cxxxvi,  pp.  199,  431,  977,  1517  (1903). 

2  Bragg:  Phil.  Mag.,  Dec.,  1904;  April,  1905. 


FIG.  46. 


234  RADIOACTIVE   TRANSFORMATIONS 

consider,  for  example,  the  outside  edge  of  the  trace  at  a  point  A. 
The  photographic  effect  at  this  edge  of  the  trace  is  due  to  the 
particles  of  lowest  velocity,  which  are  just  able  to  produce  pho- 
tographic action  at  A.  Consider  next  a  point  B  further  removed 
from  the  source.  The  a  particles,  which  produce  the  edge  of 
the  trace,  have  the  same  velocity  as  in  the  first  case ;  but  since 
they  have  had  to  travel  through  a  distance  B  R  of  air  instead  of 
A  R,  they  must  have  initially  started  with  a  greater  velocity. 
This  must  evidently  be  the  case,  since  the  a  particle  is  retarded 
in  its  passage  through  air.  The  average  velocity  of  these  a  par- 
ticles along  their  path  is  consequently  greater  than  in  the  first 
case,  and  the  outside  edge  will  be  deflected  through  a  smaller  dis- 
tance than  would  be  expected  if  the  average  velocity  were  the 
same  for  the  two  paths  A  R,  B  R.  This  will  cause  the  trace  of 
the  rays  to  show  evidence  of  steadily  increasing  radius  of  curva- 
ture as  we  proceed  from  the  source,  —  a  result  in  agreement  with 
the  observations  of  Becquerel. 

Quite  a  contrary  effect  is  produced  on  the  inside  edge  of  the 
trace,  for  this  is  produced  by  the  swiftest  a  particles  from 
radium,  viz.,  those  emitted  from  radium  C.  Since  the  velocity 
of  these  particles  decreases  in  their  passage  through  air,  the  in- 
side edge  will  show  evidence  of  decreasing  radius  of  curvature. 
This  will -have  the  effect  of  contracting  the  natural  width  of  the 
trace.  This  effect  is,  however,  small,  and  would  tend  to  be 
masked  experimentally  by  the  scattering  of  the  rays  in  their 
passage  through  air. 

There  is  another  paradoxical  effect  exhibited  by  a  complex 
/pencil  of  radium  rays.  Becquerel1  showed  that  the  outside 
edge  of  the  trace  of  rays  obtained  in  a  magnetic  field  is  unal- 
tered by  placing  absorbing  screens  over  the  radium.  In  the 
case  of  a  homogeneous  source  of  a  rays,  we  have  seen  that  the 
pencil  of  rays  suffers  a  greater  deflection  after  passing  through 
an  absorbing  screen.  The  absence  of  this  effect  in  a  complex 
pencil  of  rays  from  radium  led  Becquerel  to  believe  that  the  a 
particles  from  it  did  not  decrease  in  velocity  in  their  passage 
through  matter.  We  have  seen,  however,  that  for  each  indi- 

1  Becquerel:  Comptes  rendus,  cxli,  p.  485  (1905) ;  cxlii,  p.  365  (1906). 


PROPERTIES   OF   THE   a  RAYS  235 

vidual  product  of  radium,  the  a  particles  do  suffer  a  retardation 
of  velocity  under  such  conditions. 

The  explanation  of  this  apparent  paradox  is  simple.  The 
outside  edge  of  the  trace  of  the  complex  pencil  of  rays  is  due  to 
the  lowest  velocity  a  particles  which  are  just  able  to  produce  an 
appreciable  photographic  effect.  The  velocity  of  these  a  par- 
ticles has  been  seen  to  be  in  the  neighborhood  of  .6  V0,  where  V0 
is  the  initial  velocity  of  projection  of  the  particles  from  radium 
C.  When  an  absorbing  screen  is  placed  over  the  radium  all  the 
a  particles  suffer  a  retardation  of  velocity.  The  outside  edge 
of  the  trace  of  the  rays  is  produced  by  a  particles  of  the  same 
velocity  as  before ;  not,  however,  by  the  same  a  particles,  but  by 
another  set,  whose  velocity  has  been  diminished  to  this  minimum 
amount  in  their  passage  through  the  absorbing  screens. 

The  absence  of  increased  deflection  of  the  pencil  of  rays  by 
the  addition  of  absorbing  screens  is  thus  to  be  expected.  These 
anomalies  in  the  behavior  of  a  complex  pencil  of  rays  show  how 
necessary  it  is  to  use  a  source  of  homogeneous  rays  for  the 
investigation  of  the  properties  of  the  a  rays. 

An  account  of  these  peculiarities  of  a  complex  pencil  of  a 
rays  has  been  given  in  some  detail,  partly  because  of  their  great 
interest,  and  partly  because  the  explanation  of  the  effects  has 
been  a  subject  of  some  discussion. 

ABSORPTION  OF  THE  a  RAYS 

It  was  early  recognized  that  the  a  particles  were  stopped  in 
their  passage  through  a  few  centimetres  of  air  or  by  a  few 
thicknesses  of  metal  foil.  On  account  of  the  weak  ionization 
produced  by  uranium  and  thorium,  it  was  not  at  first  possible 
to  work  with  narrow  cones  of  a  rays ;  but  experiments  were 
made  with  a  large  area  of  radioactive  matter  spread  uniformly 
over  a  plate.  The  saturation  current  was  measured  between 
this  plate  and  another  plate  placed  parallel  to  it  at  a  distance  of 
several  centimetres.  As  successive  layers  of  aluminium  or  other 
metal  foil  were  placed  over  the  active  matter,  the  ionization  cur- 
rent was  found  to  fall  off  approximately  according  to  an  expo- 
nential law  with  the  thickness  of  the  screen.  A  thick  layer  of 


236  RADIOACTIVE   TRANSFORMATIONS 

radioactive  matter  was  generally  employed,  and  in  the  case  of 
radium  the  exponential  law  appeared  to  hold  fairly  accurately 
over  a  considerable  range. 

Some  experiments  on  the  absorption  of  the  a  rays  from  an 
active  preparation  of  polonium  were  made  in  a  different  way  by 
Mme.  Curie.  The  rays  from  the  polonium  passed  through  a 
hole  in  a  metal  plate  covered  with  a  wire  gauze,  and  the  ioni- 
zation  current  was  measured  between  this  plate  and  a  parallel 
insulated  plate  placed  3  cms.  above  it.  No  appreciable  current 
was  observed  when  the  polonium  was  4  cms.  below  the  alumin- 
ium window,  but  as  this  distance  was  diminished,  the  current 
increased  very  rapidly,  so  that  for  a  small  variation  of  distance 
there  was  a  large  alteration  in  the  ionization  current. 

This  rapid  increase  of  the  current  indicated  that  the  ioniz- 
ing property  of  the  a  rays  ceased  suddenly  after  traversing  a 
definite  distance  in  air.  By  adding  a  layer  of  foil  over  the 
polonium  this  critical  distance  was  diminished. 

The  observed  fact  that  the  ionization  current  between  two 
parallel  plates  varied  approximately  according  to  an  exponential 
law  with  the  thickness  of  the  absorbing  screen,  when  thick 
layers  of  radioactive  matter  were  employed,  tended  to  obscure 
the  true  law  of  the  absorption  of  the  a  rays ;  for  an  exponential 
law  of  absorption  had  been  observed  by  Lenard  for  the  cathode 
rays,  and  also  in  some  cases  for  the  X-rays.  In  1904,  the  ques- 
tion was  attacked  by  Bragg  and  Kleeman,1  both  on  the  theoreti- 
cal and  experimental  side,  and  the  interesting  experiments  made 
by  them  have  thrown  a  great  deal  of  fresh  light  on  the  nature 
of  the  a  rays  and  on  the  laws  of  their  absorption  by  matter. 

In  order  to  account  for  their  experimental  results,  Bragg 
formulated  a  very  simple  theory  of  the  absorption  of  the  a  rays. 
On  this  theory  all  the  a  particles  from  a  thin  layer  of  radioac- 
tive matter  of  one  kind  were  supposed  to  be  projected  with 
equal  velocities  and  to  pass  through  a  definite  distance  in  air 
before  absorption.  The  velocity  of  the  a  particle  decreased  in 
its  passage  through  air  in  consequence  of  the  expenditure  of  its 
kinetic  energy  in  ionizing  the  gas.  As  a  first  approximation,  it 

1  Bragg  and  Kleeman :  Phil.  Mag.,  Dec.,  1904;  Sept.,  1905. 


UNIVER 


PROPERTIES   OF  THE   a  RAYS 


237 


was  supposed  that  the  ionization  produced  by  a  single  a  particle 
per  centimetre  of  its  path  was  constant  for  a  certain  range,  and 
then  fell  off  very  abruptly  after  the  particle  had  traversed  a  defi- 
nite distance  of  air.  This  "  range  "  of  the  a  particle  varied  for 
each  a  ray  product  on  account  of  the  differences  in  the  initial 
velocities  of  the  a  particles  expelled  from  the  separate  products. 
If  an  absorbing  screen  were  placed  in  the  path  of  the  rays,  the 
velocities  of  the  a  particles  from  a  simple  product  were  all 
diminished  in  a  definite  ratio,  and  the  range  in  air  of  the  emerg- 
ing particles  was  reduced  by  an  amount  proportional  to  the 
thickness  of  the  screen  and  its  density  as  compared  with  air. 


FIG.  47. 


FIG.  48. 


In  a  thick  layer  of  radioactive  matter,  containing  onlyooue 
simple  source  of  rays,  the  rays  from  the  surface  will  have  the 
maximum  range  a.  Those  emerging  from  a  depth  d  of  the 
radioactive  matter  of  density  p  compared  with  air,  will  have  a 
range  in  air  of  a— p  d.  With  a  thick  layer  of  radioactive  matter, 
the  a  particles  emitted  into  the  gas  will  vary  widely  in  velocity 
and  will  have  all  ranges  in  air  between  zero  and  the  maximum 
range  a. 

Suppose  that  a  narrow  pencil  of  a  rays  from  a  simple  type  of 
radioactive  matter  R  (Fig.  47)  passes  into  the  ionization  vessel 
A  B  through  a  wire  gauze  A.  If  the  layer  of  active  matter  of 
one  kind  is  so  thin  that  the  a  rays  are  not  appreciably  retarded 
in  their  passage  normally  through  it,  the  ionization  at  different 


238  RADIOACTIVE   TRANSFORMATIONS 

distances  from  the  source  is  expressed  graphically  in  Fig.  48  by 
the  curve  A  P  M.  The  ordinates  represent  distance  from  the 
source  of  radiation  and  the  abscissae  the  ionization  produced 
in  the  vessel  The  ionization  commences  suddenly  at  A  and 
reaches  a  maximum  at  P,  when  the  rays  pass  to  the  upper  plate 
B  of  the  ionization  chamber  and  then  remains  constant  till  the 
source  is  reached. 

With  a  thick  layer,  however,  the  rays  have  all  ranges  in  air 
between  the  maximum  and  zero,  and  as  the  ionization  vessel 
approaches  the  source,  more  and  more  of  these  a  particles  pass 
into  it.  The  ionization  curve  is  consequently  then  represented 
by  a  straight  line  A  P  B. 

Theoretically,  in  order  to  obtain  such  results  a  narrow  cone 
of  rays  and  a  shallow  ionization  vessel  must  be  used.  If  the 
ionization  vessel  includes  the  whole  narrow  cone  of  rays,  at  all 
distances,  the  falling  off  of  the  intensity  of  the  radiation  accord- 
ing to  the  inverse  square  law  need  not  be  taken  into  account. 

The  experiments  of  Bragg  and  Kleeman  show  that  these  theo- 
retical conclusions  are  approximately  realized  in  practice. 

First,  let  us  consider  a  thin  layer  of  radioactive  matter  of  one 
kind.  This  was  obtained  by  evaporating  a  small  quantity  of 
radium  bromide  solution  in  a  vessel.  The  emanation  is  driven 
off  and  the  active  deposit  is  transformed  in  situ.  After  about 
three  hours  the  activity  is  due  only  to  the  a  rays  from  the 
radium  itself.  The  ionization  curve  obtained  by  Bragg  and 
Kleeman  is  shown  in  Fig.  49,  curve  A.  When  the  ionization 
chamber  is  more  than  3.5  cms.  above  the  source  only  a  slight 
current  is  observed.  At  3.5  cms.  the  current  increases  very 
rapidly  and  reaches  a  maximum  at  2.85  cms.  It  then  slowly 
falls  off  with  decreasing  distance.  The  maximum  range  of  the 
a  rays  from  radium  itself  is  consequently  3.5  cms. 

The  corresponding  curve  for  radium  C  is  shown  in  the  same 
figure,  curve  B.  This  was  examined  by  McClung,1  using  the 
methods  employed  by  Bragg  and  Kleeman.  The  radium  C  was 
deposited  as  an  extremely  thin  film  on  a  wire  by  exposure  to 
the  radium  emanation.  The  rays  from  radium  C  had  a  maxi- 

1  McClung  :  Phil.  Mag.,  Jan.,  1906. 


PROPERTIES   OF   THE   a  RAYS 


239 


mum  range  of  about  6.8  cms,  and  the  ionization  fell  off  in  a  very 
similar  way  to  that  observed  by  Bragg  for  the  radium  rays. 

In  Bragg's  experiment  the  ionization  chamber  had  a  depth  of 
2  mms.,  while  in  the  experiments  of  McClung  the  depth  was 
5  mms.  In  the  case  of  radium  C  the  ionization  is  seen  to  be 
nearly  uniform  for  a  distance  of  about  4  cms.,  and  then  to  in- 
crease rapidly,  the  maximum  ionization  being  reached  at  a  dis- 


10 


is* 


1ONIZATIQN. 

FIG.  49. 

tance  of  5.7  cms.  Allowing  for  the  fact  that  the  ionization 
chamber  had  a  sensible  depth,  and  that  a  fairly  wide  cone  of 
rays  was  employed,  it  can  be  shown  that  the  ionization  must 
increase  rapidly  at  a  distance  of  6.8  cms.,  but  not  quite  so 
rapidly  as  the  simple  theory  supposes. 

From  a  comparison  of  the  diminution  of  velocity  of  the  a  par- 
ticle from  radium  C  in  passing  through  aluminium,  it  can  be 
readily  calculated  that  the  velocity  of  the  a  particles  at  the 


240 


RADIOACTIVE   TRANSFORMATIONS 


elbow  of  the  curve  is  about  .56  of  the  initial  velocity  of  projec- 
tion. At  this  velocity  the  a  particde  appears  to  be  most  efficient 
as  an  ionizer. 

Bragg  and  Kleeman  have  examined  by  this  method  the  range 
of  the  rays  of  the  different  a  ray  products  present  in  radium  in 
radioactive  equilibrium.  A  thin  layer  of  radium  was  employed, 
and  the  ionization  curve  is  shown  in  Fig.  50. 


.6 


o  a  *  6  e  10  /a  /*  /6  19. 

i  ON/2  AT /OH. 

FIG.  50. 

The  first  a  rays  entered  the  testing  vessel  at  a  distance  of 
7.06  cms.  from  the  source.  These  rays  were  emitted  from 
radium  C,  and  have  the  greatest  range  of  all  the  rays  from  the 
radium  products.  At  a  point  b  the  curve  suddenly  turns  through 
an  angle,  showing  that  at  this  point  the  a  rays  from  another  prod- 
uct, whose  range  in  air  is  4.83  cms.,  have  entered  the  testing  ves- 
sel. There  is  a  similar  though  not  so  well  defined  break  in  the 


PROPERTIES   OF   THE   a  RAYS  241 

curve  at  d  for  a  distance  4.23  cms.,  showing  that  another  set  of 
rays  has  entered  the  testing  vessel.  The  break  at/ is  due  to 
the  appearance  of  the  rays  from  radium  itself  in  the  vessel. 
We  may  conclude  from  these  results  that  the  a  particles  from 
radium  have  a  range  of  3.5  cms.  in  air,  and  those  from  radium  C 
a  range  of  7.06  cms.  The  ranges  4.23  and  4. 83  cms.  belong  to 
the  emanation  and  to  radium  A,  but  on  account  of  the  rapid 
change  of  A  it  has  not  yet  been  found  possible  to  decide  which 
of  these  two  numbers  belongs  to  the  rays  from  the  emanation  and 
which  to  those  from  radium  A. 

If  the  curve  o  a  I  is  produced  downwards  to  c,  the  curve  o  a  b  c 
represents  the  ionization  due  to  radium  C  alone  at  different 
distances  from  the  source.  Let  this  curve  be  now  added  to  itself, 
being  first  lowered  through  a  distance  2.23  cms.,  corresponding 
to  the  difference  in  range  between  7.06  cms.  and  4.83  cms. 
The  new  curve  b  d  e  lies  accurately  along  the  experimental 
curve  b  d.  If  the  curve  be  again  lowered  through  a  distance 
6.0  mms,  corresponding  to  the  difference  of  range  for  the  next 
products,  and  a  similar  addition  be  performed,  the  resulting 
curve  dfg  again  lies  on  the  experimental  curve.  Finally  if 
the  curve  is  lowered  through  7.3  mms.,  it  is  similarly  found 
that  the  theoretical  curve  lies  on  the  experimental  curve/  h  k. 

Knowing  the  ionization  curve  of  one  product,  the  experi- 
mental curve  for  the  combined  products  caji  thus  be  built  up 
from  it  in  a  very  simple  way.  Such  a  result  shows  clearly  that, 
allowing  for  the  differences  in  the  initial  velocities  of  projection, 
the  ionization  curves  for  radium  and  each  of  its  products  are 
identical.  It  also  shows  that  the  same  number  of  a  particles 
are  projected  per  second  from  each  of  the  a  ray  products.  This 
result  follows  from  the  disintegration  theory  if  the  various 
products  are  successive. 

The  results  of  Bragg  and  Kleeman  have  thus  confirmed  in  a 
novel  and  striking  way  the  theory  of  successive  changes,  initially 
developed  from  quite  distinct  considerations.  They  show  that 
the  products  are  successive,  for  otherwise  the  experimental 
curve  could  not  be  built  up  from  consideration^  of  the  ioniza- 
tion curve  of  one  product  alone. 

16 


242  RADIOACTIVE   TRANSFORMATIONS 

We  may  thus  conclude  from  this  evidence  that  radium  A  and 
C  are  true  successive  products,  although  it  is  difficult  to  test 
this  point  satisfactorily  by  direct  experiment.  The  results  also 
indicate  that  the  a  particles  from  all  products  are  identical  in 
all  respects  except  velocity  —  a  result  confirmed,  as  we  have 
seen,  by  direct  measurements. 

The  method  developed  by  Bragg  and  Kleeman  thus  not  only  , 
throws  light  on  the  nature  of  the  absorption  of  a  rays,  but  indi-  J 
rectly  affords  a  powerful  means  of  determining  the  number  of  a    ] 
ray  products  in  radioactive  matter,  even  if  chemical  methods 
should  fail  to  isolate  these  products  from  the  parent  substance. 
This  is  possible  if  the  a  particles  from  the  separate  products 
have  different  ranges  in  air.     A  series  of  breaks  in  the  ioniza- 
tion  curve  is  a  direct  indication  of  the  presence  of  a  number 
of  distinct  radioactive  substances  which  emit  a  rays.     By  this 
method,  Dr.  Hahn  has  shown  that  thorium  B,  which  was  sup- 
posed to  contain  one  product,  in  reality  contains  two.     From 
the  difficulty  of  separation  of  these  two  products  by  chemical  or 
physical  methods,  it  appears  probable  that  one  has  an  extremely 
rapid  period  of  transformation. 

We  have  so  far  considered  the  ionization  curves  for  a  thin 
layer  of  radium,  as  this  brings  out  the  essential  features  of  the 
absorption  of  a  rays  with  great  clearness.  Bragg  and  Kleeman 
have  also  determined  the  ionization  curves  for  a  thick  layer  of 
radium.  The  curve  is  shown  in  Fig.  51.  The  curve  consists 
of  a  number  of  straight  lines  meeting  at  fairly  sharp  angles. 
Above  Q  the  ionization  is  due  to  the  rays  from  radium  C.  At 
Q  the  a  particles  from  the  product  of  range  about  4.8  cms. 
enter  the  ionization  chamber,  and  the  curve  starts  off  at  a  sharp 
angle.  A  similar  break  is  observed  at  R  and  S  when  the  a 
particles  from  the  other  two  products  enter  the  ionization  cham- 
ber. The  slopes  of  the  curves  P  Q,  Q  R,  R  S,  S  T,  are  very 
nearly  in  the  ratio  of  1,  2,  3,  and  4,  —  a  result  to  be  expected 
from  the  simple  theory. 

Experiments  were  also  made  by  Bragg  and  Kleeman  on  the 
absorption  of  the  a  rays  by  thin  metallic  layers  and  by  other 
gases  besides  air.  The  effect  of  placing  a  uniform  absorbing 


PROPERTIES   OF   THE   a  RAYS 


248 


screen  over  a  thin  layer  of  radioactive  matter  is  to  depress  the 
ionizatioii  curve  by  the  same  amount  throughout  its  whole 
course.  For  example,  the  loss  of  range  for  an  absorbing  screen 
of  silver  foil,  whose  weight  per  unit  area  was  .00967  grams,  was 
equal  to  that  for  a  stratum  of  air  of  thickness  3.35  cms.  and 


+5 
.3.5 

Pt.. 

\ 

\ 

\ 

\ 

s 

\ 

N 

x 

> 

3.               f.              6 

(P. 

?..              Q. 

~T.  +j-  <0.  ..  t- 

fl^te  o^  disc/icxrge  in  dr^itrary  5C<i/t 
FIG.  61. 

whose  weight  per  unit  area  was  .00402  gram.  The  ratio  of 
these  two  weights  is  2.41,  showing  that  the  stopping  power  of 
silver  is  2.41  times  greater  than  would  be  expected  on  a  simple 
density  law.  An  examination  of  a  number  of  metals  showed 
that  the  stopping  power  was  approximately  proportional  to  the 


244  RADIOACTIVE   TRANSFORMATIONS 

square  root  of  the  atomic  weights.  A  similar  law  was  found  to 
hold  for  gases  over  a  considerable  range  of  density.  This  rela- 
tion is  most  remarkable,  and  indicates  that  the  absorption  of 
energy  in  the  atom  is  proportional  to  the  square  root  of  its 
atomic  weight.  It  is  known  that  for  simple  gases  like  hydro- 
gen, oxygen,  and  carbon  dioxide,  the  total  number  of  ions  pro- 
duced by  complete  absorption  of  a  rays  of  given  intensity  is 
nearly  the  same,  suggesting  that  the  same  energy  is  required  in 
each  case  to  produce  an  ion.  If  the  stopping  power  of  any  gas 
is  mainly  governed  by  the  energy  used  up  in  producing  ions,  the 
results  obtained  by  Bragg  and  Kleernan  indicate  that  on  an 
average  four  times  as  many  ions  are  produced  by  the  passage 
of  an  a  particle  of  given  velocity  through  an  atom  of  oxygen  as 
in  its  passage  through  an  atom  of  hydrogen.  This  does  not  neces- 
sarily assert  that  each  atom  of  the  gas  in  the  path  of  the  a  par- 
ticle is  ionized,  but  is  supposed  to  be  an  average  result  when  a 
larger  number  of  atoms  is  considered.  At  the  same  time,  there 
is  strong  evidence  that  the  number  of  ions  produced  by  an  a, 
particle  in  air  is  at  least  as  great  as,  if  not  greater  than,  the 
number  of  molecules  with  which  it  collides.  We  are  thus 
driven  to  suppose  either  that  the  a  particle  is  able  to  produce 
more  than  two  ions  out  of  each  molecule  of  a  heavy  gas,  or  that 
the  sphere  of  action  of  the  a  particle  is  greater  in  a  dense  than 
in  a  light  gas. 

Whatever  may  be  the  conclusions  to  be  drawn  from  the 
experiments,  they  certainly  serve  to  show  that  there  is  some 
fundamental  connection  between  ionization  and  the  atomic 
weight  of  different  elements. 

CHARGE  CARRIED  BY  THE  a  RAYS 

We  have  seen  that  the  a  particle  is  deflected  in  a  magnetic  or 
electric  field  as  if  it  carried  a  positive  charge  of  electricity.  It 
wasx  early  observed  that  the  /3  particles  from  radium  carried 
with^hem  a  negative  charge,  and  that  the  radium  from  which 
they  were  expelled  gained  a  positive  charge.  This  property  of 
radium  is  illustrated  in  a  striking  manner  in  a  simple  apparatus 
devised  by  Strutt,  known  as  the  "radium  clock."  Two  gold 


PROPERTIES    OF   THE   a  RAYS  245 

leaves  are  in  metallic  connection  with  an  insulated  tube  con- 
taining radium,  and  the  whole  is  placed  in  a  vessel  exhausted  to 
a  low  vacuum.  The  /3  particles  are  fired  through  the  radium 
tube,  carrying  with  them  a  negative  charge,  and  leave  behind  an 
equal  positive  charge.  The  leaves  gradually  diverge  with  posi- 
tive electricity,  and  by  a  suitable  contact  are  made  to  discharge 
automatically  after  a  certain  divergence  has  been  reached.  This 
process  of  charging  and  discharging  continues  indefinitely,  or  at 
least  as  long  as  the  radium  itself  will  last.  Using  30  milligrams 
of  radium  bromide,  the  leaves  may  be  made  to  pass  through  the 
cycles  of  charge  and  discharge  several  times  a  minute. 

If  a  rod  or  plate,  covered  with  a  thin  film  of  radium,  which 
has  been  heated  to  get  rid  of  the  /3  and  7  rays,  is  exposed  in  a 
similar  way,  no  such  charging  action  is  observed,  however  good 
a  vacuum  is  produced.  If  the  insulated  plate  is  charged  either 
positively  or  negatively,  the  charge  is  rapidly  lost. 

Experiments  of  this  kind  are  most  simply  made  with  a  plate 
coated  with  a  thin  film  of  radiotellurium  (radium  F).^  This  sub- 
stance has  the  advantage  of  emitting  a  rays  but  not  ft  rays.  The 
reason  for  the  failure  to  detect  the  charge  carried  by  the  a  rays 
in  the  earlier  experiments  was  made  clear  by  an  investigation 
of  J.  J.  Thomson.  He  showed  that  such  an  active  plate  emit- 
ted in  addition  to  the  a  particles  a  large  number  of  slow  mov- 
ing electrons,  which  had  very  little  power  of  penetration,  and 
moved  with  so  slow  a  velocity  that  they  could  readily  be  turned 
back  or  bent  from  their  course  by  electric  or  magnetic  fields. 
The  presence  of  a  large  number  of  these  negatively  charged 
particles  under  ordinary  conditions  completely  masks  the  charge 
carried  by  the  a  particles.  The  effect  of  these  slow  moving 
electrons,  however,  can  be  almost  entirely  eliminated  by  apply- 
ing a  strong  magnetic  field  parallel  to  the  plane  of  the  active 
plate.  The  electrons  emitted  from  the  plate  then  describe  a 
curved  path  in  the  magnetic  field  and  return  to  the  plate  from 
which  they  set  out.  Under  such  conditions,  in  a  highly  ex- 
hausted vessel,  it  can  be  shown  that  the  plate  acquires  a  negative 
charge,  while  a  body  on  which  the  a  particles  impinge  receives 
a  positive  charge. 


246  RADIOACTIVE   TRANSFORMATIONS 

Such  results  show  clearly  that  the  a  particles  are  expelled 
with  a  positive  charge,  but  that  they  are  always  accompanied  by 
a  large  number  of  slow  moving  electrons.  These  electrons  ap-  v 
pear  to  be  a  type  of  secondary  radiation  set  up  by  the  escape  of 
the  a  particles  from  the  active  matter  and  by  the  matter  on 
which  they  impinge.  Their  presence  has  been  noted  not  only 
in  radiotellurium  but  also  in  radium  itself,  in  its  emanation,  and 
in  the  emanation  from  thorium.  These  electrons  appear  to  be 
a  necessary  accompaniment  of  the  emission  of  a  particles,  but 
must  not  be  confused  with  the  /3  rays  proper,  which  are  pro- 
jected at  a  much  greater  speed  and  have  a  much  greater  pene- 
trating power.  By  employing  a  magnetic  field  to  get  rid  of  the 
disturbance  caused  by  the  slow  moving  electrons,  the  writer 
determined  the  charge  carried  by  the  a  rays  from  a  thin  film  of 
radium  spread  uniformly  on  a  metal  plate.  Knowing  the  quan- 
tity of  radium  on  the  plate,  it  was  deduced  that  6.2  x  1010  a 
particles  were  projected  per  second  from  a  gram  of  radium  at  its 
minimum  activity.  For  radium  in  equilibrium,  which  contains 
four  a  ray  products,  the  corresponding  number  is  2.5  x  1011. 
These  calculations  are  based  on  the  assumption  that  each  a  par- 
ticle carries  a  single  ionic  charge  of  value  3.4  x  1010  electro- 
static units.  If  the  a  particle  carries  double  this  charge  the 
number  expelled  is  only  one  half  of  the  above. 

By  measurements  of  the  charge  carried  by  the  a  rays  from 
radium  C,  obtained  on  a  lead  rod  by  exposure  to  the  radium 
emanation,  it  was  calculated  that  7.3  x  1010  /3  particles  are  ex- 
pelled per  second  from  one  gram  of  radium.  Recently  Schmidt 
has  shown  that  the  supposed  rayless  product  radium  B,  as  well 
as  radium  C,  emits  ft  particles,  but  that  these  have  much  smaller 
penetrating  power.  If  equal  numbers  of  /3  particles  are  ex- 
pelled per  second  from  radium  B  and  C  the  number  for  each  0 
ray  product  per  gram  of  radium  is  3.6  x  1010. 

McClelland  has  shown  that  a  strong  secondary  radiation  is  v 
set  up  by  the  impact  of  /3  particles  on  lead.     It  is  thus  probable 
that  the  number  3.6  x  1010  is  too  high,  for  the  /3  particles  fired 
into  the  lead  give  rise  to  secondary  /3  particles  whose  charge  is 
measured  with  that  of  the  primary  /3  particles.     If  each  a  par- 


PROPERTIES   OF   THE   a  RAYS  247 

tide  carries  twice  the  charge  of  the  ft  particle,  the  number  of  £ 
particles  expelled  per  second  for  each  product  in  a  gram  of 
radium  should  be  8.1  x  1010.  Although  it  is  difficult  to  draw 
any  very  definite  conclusions  from  such  comparisons,  the  evi- 
dence is  in  agreement  with  the  view  that  in  the  product  radium 
C  which  emits  a  and  /3  rays,  the  number  of  a  and  ft  particles 
emitted  per  second  is  the  same,  while  the  charge  carried  by  the 
a  particle  is  twice  that  carried  by  the  ft  particle. 

HEATING  EFFECT  OF  THE  a  RAYS 

In  1903,  Curie  and  Laborde1  made  the  striking  discovery 
that  radium  was  always  hotter  than  the  surrounding  medium, 
and  radiated  heat  at  a  constant  rate  of  about  100  gram  calories 
per  hour  per  gram.  The  question  immediately  arose  as  to 
whether  this  phenomenon  involved  some  new  scientific  prin- 
ciple, or  whether  it  was  merely  a  secondary^effect  due  to  the 
bombardment  of  radium  by  its  own  a  particles. 

Since  the  a  particles  have  large  kinetic  energy  and  are  very 
easily  stopped  by  matter,  most  of  those  produced  within  the 
radium  do  not  emerge,  but  are  stopped  by  the  radium  itself,  and 
their  energy  of  motion  is  transformed  into  heat  in  situ.  In 
measurements  of  its  heating  effect  the  radium  is  enclosed  in  a 
vessel  of  sufficient  thickness  to  absorb  all  the  a  rays  emitted 
from  the  surface.  It  is  consequently  not  necessary  to  make  any 
correction  for  the  a  particles  which  escape  from  the  radium  itself. 
It  thus  appeared  probable  that  the  heating  effect  of  radium 
might  result  largely  from  the  bombardment  of  the  radium  itself 
by  the  a  particles  produced  within  it. 

Rutherford  and  Barnes  2  made  a  number  of  experiments  to 
throw  light  on  this  subject.  The  heating  effect  of  about  30 
milligrams  of  radium  bromide  was  first  measured  in  a  simple 
form  of  air  calorimeter  and  was  found  to  correspond  to  about  100 
gram  calories  per  hour  per  gram.  The  radium  was  then  heated 
to  a  sufficient  temperature  to  drive  off  the  emanation,  which 
was  then  condensed  in  a  small  glass  tube  by  means  of  liquid 

1  Curie  and  Laborde  :  Comptes  rendus,  cxxxvi,  p.  673  (1904). 

2  Rutherford  and  Barnes :  Phil.  Mag.,  Feb.,  1904. 


248 


RADIOACTIVE   TRANSFORMATIONS 


air,  and  the  tube  sealed.  The  variation  in  the  heating  effect  of 
the  radium  so  treated  and  of  the  emanation  tube  were  then  sepa- 
rately examined.  After  removal  of  the  emanation,  the  heating 
effect  of  the  radium  fell  rapidly  in  the  course  of  about  three 
hours  to  27  per  cent  of  its  maximum,  and  then  slowly  increased 
again,  finally  reaching  its  old  value  after  a  month's  interval. 

The  heating  effect  of  the  emanation  tube  varied  in  exactly 
the  opposite  way,  for  it  increased  to  a  maximum  in  about  three 


In'it 


I  Heaf  Ei  fission 
of  Radi\ 


/•« 


o      9 
<o 


O  60  /6O  240  320 

Xours 
FIG.  52. 

Variation  of  heating  effect  of  radium  after  removal  of  its  emanation  and  vari- 
ation of  that  of  the  emanation  and  its  products. 

hours,  when  it  was  equal  to  about  73  per  cent  of  the  heating 
effect  of  the  radium.  It  then  gradually  decreased  according  to 
an  exponential  law,  falling  to  half  value  in  about  four  days. 
The  curves  of  recovery  of  the  heating  effect  of  the  radium  and 
of  the  loss  of  heating  effect  of  the  emanation  tube  are  shown  in 
Fig.  52.  Within  the  limit  of  experimental  error,  the  sum  of  the 
heating  effects  of  the  radium  and  emanation  tube  was  equal  to 
that  of  the  radium  in  radioactive  equilibrium.  Allowing  for  the 
fact  that  6  per  cent  of  the  emanation  was  not  removed  by  the 


PROPERTIES   OF   THE   a  RAYS 


249 


heating,  it  is  seen  that  only  23  per  cent  of  the  heating  effect  is 
due  to  radium  itself,  and  the  other  77  per  cent  to  the  emanation 
and  its  products. 

The  decay  and  recovery  curves  of  the  heating  effect  are  iden- 
tical within  the  limit  of  experimental  error  with  the  correspond- 
ing curves  of  decay  and  recovery  of  activity  measured  by  the  a 
rays.  Such  a  result  indicated  that  the  heating  effect  is  a  meas- 
ure of  the  kinetic  energy  of  the  a  particles,  for  radium  has  an  a 


100; 


\ 


3,9 


60, 


UO  M1NUTC4 


FIG.  53 


ray  activity  of  about  25  per  cent  of  its  maximum  when  the 
emanation  and  its  products  are  removed,  while  the  /3  and  7  ray 
activity  practically  disappears.  In  order  to  test  this  point  still 
further,  the  distribution  of  the  total  heating  effect  of  the  emana- 
tion tube  between  the  emanation  and  its  further  products  was 
determined.  After  measuring  the  heating  effect  of  the  emana- 
tion tube,  the  end  of  the  tube  was  broken  and  the  emanation 
was  entirely  removed.  Ten  minutes  later  the  heating  effect  of 
the  tube  had  dropped  to  48  per  cent,  and  it  then  diminished 


250  RADIOACTIVE   TRANSFORMATIONS 

steadily  to  zero.  The  curve  of  decrease  of  heating  effect  is 
shown  in  Fig.  53.  After  ten  minutes  the  curve  follows  the 
activity  decay  curve  fairly  closely.  After  removal  of  the 
emanation  the  heating  effect  of  the  tube  is  due  to  the  active 
deposit  consisting  of  radium  A,  B,  and  C,  which  remains  be- 
hind. Since  A  loses  its  activity  according  to  a  three  minute 
period,  it  is  not  possible  to  follow  the  variation  of  its  heating 
effect.  After  fifteen  minutes  the  heating  effect  must  be  due 
entirely  to  radium  B  and  C.  It  is  not  easy  to  decide  experi- 
mentally whether  the  rayless  product  radium  B  supplies  an  ap- 
preciable proportion  of  the  heating  effect,  but  from  the  absence 
of  a  rays,  its  heating  effect  is  probably  very  small  compared  with 
that  of  C. 

The  curve  of  increase  of  heating  effect  (Fig.  53)  of  a  tube  into 
which  the  emanation  has  been  introduced  is  complementary  to 
that  of  the  curve  of  decrease.  Such  a  relation  is  to  be  expected. 
The  fact  that  the  heating  effect  falls  off  according  to  the  period 
of  each  a  ray  product  shows  that  the  heat  emission  of  radium 
and  its  products  is  mainly  a  consequence  of  the  expulsion  of  a 
rays. 

It  was  deduced  from  the  experiments  that  about  23  per  cent 
of  the  heating  effect  was  due  to  radium  alone,  32  per  cent  to 
radium  C,  and  45  per  cent  to  the  emanation  and  radium  A  to- 
gether. On  account  of  the  rapid  rate  of  the  change  of  A,  its 
heating  effect  cannot  be  disentangled  from  that  of  the  emana- 
tion. Direct  experiment  has  shown  that  not  more  than  one 
or  two  per  cent  of  the  heat  emission  of  radium  is  due  to  the  0 
or  7  rays  even  when  they  are  completely  absorbed  by  a  lead 
envelope. 

We  shall  now  consider  the  important  question  as  to  whether 
the  energy  of  motion  of  the  a  particles  ejected  from  radium  and 
its  products  is  sufficient  to  account  for  the  heating  effect  ob- 
served. The  kinetic  energy  of  an  a  particle  of  mass  m  moving 
with  a  velocity  v  is  \mv*.  The  relative  kinetic  energies  of 
the  a  ray  particles  from  each  of  the  a  ray  products  can  at  once 
be  determined  if  their  relative  velocities  are  known.  These 
velocities  have  not  yet  been  directly  measured  for  the  rays  from 


PROPERTIES   OF   THE   a  RAYS  251 

each  product,  but  they  can  readily  be  deduced  from  the  ranges 
of  the  a  particles  in  air.  For  example,  the  velocity  of  the  a 
particle  from  radium  itself  is  equal  to  that  of  the  particle  from 
radium  C  after  the  latter  has  passed  through  a  distance  of  air 
equal  to  the  difference  of  the  ranges  of  the  two  a  particles  in  air. 
This  difference  is  3.5  cms.,  which  corresponds  to  6.7  layers  of 
aluminium  foil  of  the  same  thickness  as  that  employed  in  the 
experimental  results  tabulated  on  page  225.  In  this  way,  taking 
the  kinetic  energy  of  the  a  particle  from  radium  C  as  100,  it 
can  readily  be  deduced  that  the  kinetic  energies  of  the  a  par- 
ticles of  range  4.8  and  4.3  cms.  are  74  and  69  respectively.  For 
the  a  rays  from  radium  itself  of  range  3.5  cms.,  the  energy  is 
58.  Since  the  products  in  radium  are  successive,  the  same  num- 
ber of  a  particles  is  expelled  per  second  from  each  product. 
Taking  the  kinetic  energy  of  the  a  particle  as  a  comparative 
measure  of  its  heating  effect,  it  can  at  once  be  deduced  from 
these  numbers  that  19  per  cent  of  the  total  heating  effect  should 
be  produced  by  radium  itself,  48  per  cent  by  radium  A  and  the 
emanation  together,  and  33  per  cent  by  radium  C.  The  corre- 
sponding values  obtained  by  direct  measurement  of  the  heating 
effect  are  23,  45,  and  32  per  cent  respectively.  The  agreement 
between  theory  and  experiment  is  thus  fairly  good. 

Now,  on  the  assumption  that  the  a  particle  carries  an  ionic 
charge  of  1.13  x  10~20  electromagnetic  units,  it  has  been  experi- 
mentally found  that  each  of  the  a  ray  products  present  in  one 
gram  of  radium  product  expels  6.2  x  1010  a  particles  per  second. 
The  kinetic  energy  of  the  a  particle  from  radium  C  is  %mv2. 
Substituting  the  known  values  e/m  =  5  x  103,  and  v  =  2.0  x  109, 
ande  =  1.13  X  10~20,  the  kinetic  energy  is  seen  to  be  4.5  x  10"6 
ergs.  The  value  of  the  kinetic  energy  of  radium  C  deduced 
in  this  way  is  independent  of  whether  the  a  particle  carries  one 
or  two  ionic  charges.  The  kinetic  energy  of  the  a  particles  of 
radium  C  expelled  per  second  from  one  gram  of  radium  is  thus 
2.79  x  105  ergs.  The  heating  effect  of  the  a  particles  from 
radium  C  per  hour  per  gram  of  radium  is  consequently  24  gram 
calories.  The  amount  experimentally  observed  is  32  gram  cal- 
ories per  hour. 


252  RADIOACTIVE   TRANSFORMATIONS 

Considering  the  experimental  difficulty  of  accurately  deter- 
mining the  number  of  a  particles  expelled  from  radium  per  sec- 
ond, the  agreement  of  experiment  and  calculation  is  remarkably 
good,  and  shows  clearly  that  the  greater  part  of  the  heat  emis- 
sion of  radium  is  due  to  self-bombardment  by  its  own  a  par- 
ticles. It  is  possible  that  a  small  fraction  of  the  heat  emitted 
by  radium  may  be  due  to  the  energy  liberated  in  consequence  of 
the  rearrangement  of  the  atom  after  the  violent  expulsion  of  the 
a  particle,  but  it  appears  probable  that  this  would  be  small  com- 
pared with  the  energy  of  motion  of  the  a  particle  itself. 

The  conclusion  that  the  heating  effect  of  radium  and  its  prod- 
ucts is  a  measure  of  the  energy  of  the  a  particles  produced  by 
it  applies  also  equally  well  to  the  other  radioelements  which 
emit  a  rays.  We  should  thus  expect  thorium,  uranium,  and 
actinium  to  emit  heat  at  a  rate  approximately  proportional  to 
their  a  ray  activities.  Pegram  has  examined  this  question  for 
thorium  and  found  evidence  of  a  rate  of  heat  emission  about 
that  to  be  expected  from  its  activity  compared  with  that  of 
radium.  Every  substance  that  emits  a  particles  must  thus  also 
emit  heat  at  a  rate  proportional  to  the  product  of  the  number  of 
a  particles  produced  per  second  and  the  average  kinetic  energy 
of  each  a  particle. 

The  enormous  heating  effect  of  the  radium  emanation  com- 
pared with  the  quantity  of  matter  involved  has  already  been 
discussed  on  page  91.  The  rapidly  changing  products  like  the 
actinium,  and.  thorium  emanations,  and  radium  A,  must  initially 
emit  heat  at  an  enormous  rate  compared  with  the  quantity  of 
matter  present.  For  example,  the  actinium  emanation  which 
has  a  period  of  3.9  seconds  must,  weight  for  weight,  emit  heat 
at  about  800,000  times  the  rate  of  the  radium  emanation.  The 
average  duration  of  the  time  during  which  heat  is  emitted  is 
correspondingly  less. 

GASES   EVOLVED  FROM  RADIUM 

We  have  already  seen  that  helium  is  produced  in  small 
amounts  from  radium.  Giesel,  Runge,  and  Bodlander  observed 
that  radium  solutions  produced  a  considerable  quantity  of  hydro- 


PROPERTIES   OF   THE  a  RAYS  253 

gen  and  oxygen.  Ramsay  and  Soddy  found  that  50  milligrams  f 
of  radium  bromide  in  solution  evolved  about  0.5  c.c.  of  mixed  < 
gases  per  day.  About  28.9  per  cent  of  this  consisted  of  oxygen 
and  the  rest  of  hydrogen/TKe^  hydrogen  is  thus  present  in  a 
slight  excess  compared  with  that  obtained  by  the  decomposition 
of  water,  and  no  satisfactory  explanation  of  this  excess  has  yet 
been  given.  It  may  possibly  be  due  to  oxidation  of  the  radium 
bromide  to  radium  bromate.  Ramsay  showed  that  the  radium 
emanation  mixed  with  water  produced  hydrogen  and  oxygen, 
and  that  after  explosion  of  the  mixed  gases  no  observable 
bubble  of  gas  remained.  This  evolution  of  gas  proceeds  at  a 
constant  rate,  and  must  be  the  result  of  the  action  of  the  a 
radiations  in  decomposing  the  water  molecules.  One  gram  of 
radium  bromide  in  equilibrium  would  produce  about  10  c.c.  of 
hydrogen  and  oxygen  per  day.  The  energy  required  to  disso- 
ciate the  corresponding  amount  of  water  per  day  is  about  20 
gram  calories,  or  less  than  2  per  cent  of  the  total  kinetic  energy 
of  the  a  particles  produced  by  radium. 

In  order  to  generate  10  c.c.  of  hydrogen  and  oxygen  per  day 
'by  electrolysis  a  steady  current  of  .00067  amperes  is  required. 
Now  it  is  found  experimentally  that  the  maximum  ionization 
current  in  air  due  to  one  gram  of  radium  bromide  in  equilib- 
rium, spread  in  a  thin  film,  is  .0013  amperes,  or  about  twice 
the  current  required  to  produce  the  amount  of  hydrogen  and 
oxygen  observed. 

In  the  experiments  of  Ramsay  and  Soddy  some  of  the  emana- 
tion left  the  solution  and  collected  in  the  open  space  above  it, 
and  the  rate  of  production  of  gases  observed  is  probably  less 
than  if  the  emanation  were  all  retained  in  the  solution.  In 
addition,  the  a  rays  not  only  decompose  water  but  conversely 
cause  the  combination  of  hydrogen  and  oxygen  to  form  water. 
Taking  such  factors  into  account,  it  certainly  appears  more*  than 
a  coincidence  that  the  ionization  current  in  air  due  to  the  a 
rays  from  radium  is  of  the  right  magnitude  to  account  for  the 
observed  production  of  hydrogen  and  oxygen. 

The  gradual  loss  of  energy  of  the  a  particles  in  their  passage 
through  a  gas  appears  to  be  mainly  due  to  the  energy  absorbed 


254  RADIOACTIVE   TRANSFORMATIONS 

in  the  ionization  of  the  gas.  The  fact  that  the  stopping  power 
of  matter  in  general,  whether  solid,  liquid,  or  gaseous,  is  propor- 
tional to  the  square  root  of  the  atomic  weight,  suggests  that 
matter  of  all  kinds  is  ionized  by  the  passage  of  the  a  rays.  It  is 
thus  to  be  expected  that  about  the  same  total  number  of  ions 
would  be  produced  by  the  complete  absorption  of  the  a  rays  in 
water  as  are  produced  by  absorption  in  air.  The  appearance  of 
hydrogen  and  oxygen  in  the  radium  solution  is  undoubtedly 
mainly  a  result  of  the  ionization  of  the  water  molecules  by  the 
a  particles,  and  shows  that  the  ionization  consists  in  large  part 
in  an  actual  chemical  dissociation  of  the  water  molecules.  It 
has  been  generally  supposed  that  ionization  in  simple  gases  like 
helium,  hydrogen,  and  oxygen  is  due  to  the  expulsion  of  an 
electron  from  the  molecule.  This  may  be  the  case,  but  in  a 
complex  molecule  like  that  of  water,  the  ionization  by  the  a 
rays  consists  in,  or  at  any  rate  results  in,  an  actual  chemical  dis- 
sociation of  water  into  hydrogen  and  oxygen.  Whether  this 
dissociating  action  is  a  property  only  of  the  a  rays,  or  of  all 
powerful  ionizing  agencies,  cannot  be  discussed  at  this  point,  but 
the  evidence  certainly  suggests  that  the  ionization  of  complex 
substances  by  the  a  rays  is  very  similar  in  character  to  ionization 
of  solutions,  and  consists  in  part  in  a  chemical  dissociation  of 
the  substance. 

There  is  considerable  evidence  that  the  a  particles  produce 
chemical  action  of  various  kinds.  For  example,  the  a  rays  con- 
vert oxygen  into  ozone,  coagulate  globulin,  and  produce  chemi- 
cal changes  in  barium  platinocyanide. 

SUMMARY  OF  PROPERTIES  OF  THE  a  RAYS 

(1)  The  a  particles    from   radium,   and   probably   from   all 
radioactive   substances,  consist  of  positively  charged  atoms  of 
matter  projected  with  great  velocity. 

(2)  The  a  particles  from  radium  and  its  products  all  have 
the  same  mass,  and  are  probably  atoms  of  helium. 

(3)  Each  product  of  radium  expels  a  particles  at  a  definite 
speed,  which  is  characteristic  of  that  product,  but  varies  for 
different  products. 


PROPERTIES   OF   THE   a  RAYS  255 

(4)  The  ionizing,  photographic,  and  phosphorescent  actions 
of  the  a  rays  from  a  simple  product  all  appear  to  cease  abruptly 
when  the  velocity  of  the  a  particle  falls  below  a  certain  critical 
speed. 

(5)  The  velocity  of  projection  of   the  a  particles  increases 
steadily  for  the  successive  products  of  radium,  and  is  greatest  for 
those  emitted  from  ^adium  C.     The  maximum  velocity  is  about 
2  x  109  cms.  per  second. 

(6)  The  velocity  of  the  a  particle  is  diminished  in  its  passage 
through  matter. 

(7)  The  a  rays  from  a  thin  layer  of  any  simple  product  are 
homogeneous,  i.  £.,  they  consist  of  a  particles  all  projected  with 
the  same  velocity.     On  account  of  the  retardation  of  the  a  par- 
ticles in  passing  through  matter,  the  rays  from  a  thick  layer  of 
any  simple  radioactive  substance  are  complex,  i.  e.,  they  consist 
of  a  particles  projected  with  velocities  varying  over  a  consider- 
able range. 

(8)  The  initial  velocities  of  projection  of  the  a  particles  from 
radium  and  its  products  lies  between  109  and  2  x  109  cms.  per 
second. 

(9)  The  heating  effect  of  radium  is  a  result  of  the  bombard- 
ment of  the  radium  by  its  own  a  particles. 


CHAPTER  XI 
PHYSICAL  VIEW  OF  RADIOACTIVE  PROCESSES 

IN  the  preceding  chapters,  the  more  important  properties  of 
the  radioactive  bodies  have  been  considered,  and  it  has  been 
seen  that  the  results  obtained  receive  a  satisfactory  explanation 
on  the  view  that  the  radioactive  matter  suffers  spontaneous 
disintegration. 

We  shall  now  endeavor  to  outline  in  as  concrete  a  manner  as 
possible  the  processes  which  are  believed  to  take  place  in  the 
atoms  of  radioactive  matter,  and  in  the  medium  surrounding 
them.  Such  representations  of  the  nature  of  the  atom  and  of 
the  processes  occurring  therein  are,  in  the  present  state  of  our 
knowledge,  somewhat  speculative  and  imperfect,  but  they  are 
nevertheless  of  the  greatest  assistance  to  the  investigator  in 
providing  him  with  a  working  hypothesis  of  the  structure  of  the 
atom.  The  behavior  of  such  model  atoms  can  be  compared  with 
that  of  the  actual  atoms  of  matter  under  investigation,  and  in 
this  way  it  is  possible  to  form  gradually  a  clearer  and  more 
definite  idea  of  the  constitution  of  the  atom. 

Modern  physical  and  chemical  th»£ories  are  all  based  on  the 
assumption  that  matter  is  discontinuous  and  is  made  up  of  a 
number  of  discrete  atoms.  In  each  element  the  atoms  are  sup- 
posed to  be  all  of  the  same  mass  and  of  the  same  constitution, 
but  the  atoms  of  the  different  elements  show  well  marked  dif- 
ferences in  physical  and  chemical  behavior.  It  has  been  quite 
incorrectly  assumed  by  some  that  the  study  of  radioactive  phe- 
nomena has  tended  to  cast  doubt  on  atomic  theories.  Far  from 
this  being  the  case,  such  a  study  has  materially  strengthened,  if, 
indeed,  it  has  not  given  actual  proof  of,  the  atomic  structure  of 
matter. 

Any  one  who  has  witnessed  the  multitude  of  scintillations 
produced  on  a  zinc  sulphide  screen  by  the  a  rays  of  radium  can- 


RADIOACTIVE   PROCESSES  257 

not  fail  to  have  been  impressed  with  the  idea  that  radium  is 
sending  out  a  shower  of  small  particles.  Such  a  view  is  con- 
firmed by  direct  measurement,  for  we  know  that  the  scintilla- 
tions are  due  to  the  a  particles,  which  consist  of  minute 
material  bodies,  all  of  the  same  mass,  which  are  expelled  from 
the  radium  at  an  enormous  speed.  The  energy  of  motion  of 
each  a  particle  is  so  great  that  in  some  cases  the  impact  of  the  a 
particle  on  the  screen  is  accompanied  by  a  visible  flash  of  light. 
These  a  particles,  as  we  have  seen,  are  not  fragments  of  radium, 
but  atoms  of  helium. 

While  the  study  of  radioactivity  has  emphasized  the  ideas  of  i 
the  atomic  constitution  of  matter,  it  has  at  the  same  time  indi-  \ 
cated  that  the  atom  is  not  an  indivisible  unit  but  a  complex 
system  of  minute  particles.  In  the  case  of  the  radioactive  ele- 
ments, some  of  the  atoms  become  unstable  and  break  up  with 
explosive  violence,  expelling  in  the  process  a  portion  of  their 
mass.  Such  views  are  rather  an  extension  than  a  contradiction 
of  the  usual  chemical  theory  which  supposes  that  the  chemical 
atom  is  the  smallest  combining  unit  of  matter  in  ordinary  chemi- 
cal change.  The  atom  may  be  the  smallest  combining  unit,  and 
still  be  a  complex  system  which  cannot  be  broken  up  by  any 
physical  or  chemical  agencies  under  our  control. 

In  fact,  the  great  emission  of  energy  in  radioactive  changes 
shows  clearly  the  reason  why  chemistry  has  failed  to  break  up 
the  atom.  The  forces  which  bind  together  the  component  parts 
of  an  atom  are  so  great  that  an  enormous  concentration  of  energy 
would  be  required  to  break  up  the  atom  by  the  action  of  exter- 
nal agencies. 

The  complex  structure  of  the  atom  is  clearly  indicated  by  an 
examination  of  its  spectrum.  Under  the  stimulus  of  heat  or 
the  electrical  discharge,  the  atom  has  certain  definite  periods  of 
vibration  which  are  characteristic  of  each  particular  element. 
Even  in  the  case  of  a  light  atom  like  hydrogen,  the  great  number 
of  different  periods  of  vibration  extending  far  into  the  ultra- 
violet shows  that  the  atom  must  be  a  complex  structure  which 
is  able  to  vibrate  in  a  variety  of  ways.  The  modes  of  vibration 
of  a  hydrogen  atom  are  exactly  the  same  under  all  conditions, 

17 


258  RADIOACTIVE   TRANSFORMATIONS 

and  are  the  same,  for  example,  for  free  hydrogen  in  the  sun  and  for 
hydrogen  prepared  by  different  chemical  processes  on  the  earth. 
The  unchanging  character  of  the  spectrum  of  the  elements 
has  been  urged  by  some  as  an  objection  to  the  view  that  atoms 
suffer  disintegration.  This  objection,  however,  does  not  appear 
to 'be  a  very  weighty  one,  for  the  present  theories  of  atomic  dis- 
integration suppose  that  there  is  not  a  gradual  variation  of  the 
properties  of  the  atoms  as  a  whole,  but  a  sudden  disintegration  of 
a  minute  fraction  of  the  total  number  present,  the  rest  remaining 
quite  unchanged.  For  example,  the  spectrum  of  radium  itself 
remains  unaltered  so  long  as  any  radium  remains  untransf  ormed. 
Supposing,  however,  we  were  able  to  detect  the  spectrum  of  the 
products  mixed  with  it,  we  should  find  the  spectrum  of  radium 
in  equilibrium  to  consist  of  the  normal  radium  spectrum,  plus 
the  spectrum  of  each  of  its  products  superimposed  upon  it. 
Each  product  would  have  a  definite  and  characteristic  spectrum, 
differing  for  each  product,  and  having  no  apparent  connection 
with  that  of  the  parent  element. 

DEVELOPMENT  OF  THE  ELECTRONIC  THEORY 
OF  MATTER 

The  laws  of  electrolysis  discovered  by  Faraday  indicated  that 
each  atom  of  hydrogen  carried  an  invariable  charge,  whose  value 
e  could  be  approximately  deduced  from  data  on  the  mass  of  the 
atom.  The  oxygen  atom  always  carried  a  charge  2  e  and  an 
atom  of  gold  3  e,  and,  generally,  the  ions  of  different  elements  in 
solution  carried  charges  which  were  integral  multiples  of  the 
charge  carried  by  the  hydrogen  atom.  In  no  case  was  any 
atom  found  to  carry  a  charge  less  than  e.  The  idea  thus  arose 
that  the  charge  carried  by  the  hydrogen  atom  was  the  smallest 
unit  of  electricity,  and  was  not  capable  of  further  subdivision. 
Such  a  conception  was  practically  equivalent  to  an  atomic 
theory  of  electricity. 

Theories  of  atomic  constitution  were  advanced  in  which  it 
was  supposed  that  the  atom  consisted  of  a  number  of  charged 
ions  in  motion.  Such  theories,  of  which  the  most  notable 
exponents  were  Larmor  and  Lorentz,  were  primarily  advanced 


RADIOACTIVE   PROCESSES  259 

to  explain  the  mechanism  of  atomic  radiation.  More  physical 
definiteness  was  given  to  such  theories  by  the  discovery  of  J.  J. 
Thomson  that  the  cathode  rays  consisted  of  a  flight  of  particles, 
whose  apparent  mass  was  only  about  1/1000  that  of  the  hydro- 
gen atom.  These  "  corpuscles  "  or  "  electrons  "  were  found  to 
be  emitted  from  substances  under  a  variety  of  conditions.  Not 
only  were  they  obtained  by  means  of  the  electric  discharge  in  a 
vacuum  tube,  but  also  from  a  white  hot  carbon  filament  and 
from  a  zinc  or  other  metal  plate  exposed  to  the  action  of  ultra- 
violet light.  They  were  found  to  be  spontaneously  emitted 
from  the  radioactive  bodies,  with  velocities  in  many  cases  much 
greater  than  those  attained  in  a  vacuum  tube. 

At  the  same  time  the  discovery  by  Zeeman  of  the  effect  of  a 
magnetic  field  on  the  period  of  the  light  vibrations  indicated 
that  the  vibrating  system  consisted  of  negatively  charged  par- 
ticles, whose  mass  was  about  the  same  as  that  of  the  electron 
set  free  in  a  vacuum  tube.  Such  results  indicated  that  the 
electron  was  a  constituent  of  all  matter,  and  escaped  from  it 
under  a  variety  of  conditions. 

It  was  at  first  assumed  as  the  simplest  hypothesis  that  the 
electron  consisted  of  a  material  particle  of  mass  about  1/1000 
that  of  the  hydrogen  atom,  and  carrying  the  same  charge  as  a 
hydrogen  atom  in  the  electrolysis  of  water.  Theory  had  long 
before  shown  that  a  moving  charge  possessed  electrical  mass  in 
virtue  of  its  motion.  The  theory  indicated  that  this  electrical 
mass  should  be  constant  for  slow  speeds,  but  should  increase 
rapidly  as  the  velocity  of  light  is  approached.  In  order  to  test 
this  theory  definitely  it  was  necessary  to  determine  the  value  of 
e/m  for  the  electrons  moving  at  velocities  closely  approaching 
that  of  light. 

Radium  proved  to  be  an  ideal  source  of  electrons  for  such  an 
experiment,  since  it  expels  a  particles  over  a  wide  range  of 
speed,  the  velocity  of  some  of  them  being  very  nearly  equal  to 
that  of  light.  As  we  have  seen,  Kaufmann  measured  the  ve- 
locity and  value  of  e/m  for  the  electrons  from  radium,  and 
definitely  showed  that  the  apparent  mass  of  the  electron  in- 
creased with  its  velocity.  By  comparison  of  theory  with  experi- 


260  RADIOACTIVE   TRANSFORMATIONS 

merit,  it  was  found  that  the  mass  of  the  electron  was  purely 
electrical  in  origin,  and  that  there  was  no  necessity  to  assume 
that  the  charge  was  distributed  over  a  material  nucleus.  We 
thus  arrive  at  the  remarkable  conclusion  that  the  particles  of  the 
cathode  stream  and  the  /3  particles  of  radium  are  not  matter  at 
all  in  the  ordinary  sense,  but  disembodied  electrical  charges 
whose  motion  confers  on  them  the  properties  of  ordinary  mass. 
It  has  already  been  pointed  out  (page  11)  that  ordinary  mass 
itself  may  possibly  be  explained  purely  as  a  result  of  electricity 
in  motion.  In  order  to  account  for  the  observed  magnitude 
of  the  mass  of  the  electron  at  different  speeds,  it  is  necessary 
to  suppose  that  the  electricity  is  distributed  over  a  surface  of 
minute  area  or  throughout  a  minute  volume. 

Taking  as  the  simplest  assumption  that  this  surface  is  spherical 
in  shape,  it  is  necessary  to  suppose  that  the  radius  of  the  sphere 
on  which  the  charge  is  distributed  is  about  10~~13  cms.  Now, 
from  a  variety  of  considerations,  it  can  be  calculated  that  the 
radius  of  an  atom  is  about  10~8  cms.,  or  rather  that  the  sphere  of 
action  of  atomic  forces  extends  from  the  centre  of  the  atom  over 
about  this  distance.  We  thus  see  that  if  an  atom  were  magni- 
fied so  as  to  be  represented  by  a  sphere  of  100  metres  radius,  the 
radius  of  an  electron  would  be  only  one  millimetre.  Conse- 
quently, if  we  suppose  that  the  hydrogen  atom  consists  of  a 
thousand  electrons  free  to  move  within  the  dimensions  of  the 
atom,  the  electrons  would  be  so  sparsely  distributed  that  they 
would  occupy  rather  than  fill  the  space  within  the  atom,  and 
would  only  occasionally  interfere  with  the  freedom  of  one  an- 
other's individual  motion. 

Most  of  the  magnetic  and  electric  field  surrounding  an  electron 
in  motion  lies  close  to  its  surface.  This  must  evidently  be  the 
case,  since  the  magnitudes  of  both  these  forces  diminish  in- 
versely as  the  square  of  the  distance,  and  become  comparatively 
small  at  a  distance  from  the  electron  corresponding  to  a  few 
radii.  The  forces  due  to  an  electron  in  motion  are  thus  mostly 
confined  within  a  sphere  of  radius  about  10~12  cms.  The  direc- 
tion or  magnitude  of  the  motion  of  any  electron  would  not  be 
sensibly  disturbed  by  the  presence  of  another  unless  it  ap- 
proached within  this  small  limiting  distance. 


RADIOACTIVE   PROCESSES  261 

It  has  been  found  experimentally  that  an  ion  produced  in 
gases  by  X-rays,  or  the  rays  from  active  bodies,  carries  a  positive 
or  negative  charge  equal  in  magnitude  to  3.4  x  10~10  electro- 
static units.  The  charge  carried  by  an  ion  in  gases  is  apparently 
the  same  for  all  gases,  and  does  not  vary,  as  in  the  case  of  elec- 
trolysis, with  the  valency  of  the  atom.  The  charge  carried  by 
an  ion  has  been  shown  to  be  identical  with  that  carried  by  a 
hydrogen  atom  set  free  in  the  electrolysis  of  water. 

Although  the  charge  carried  by  an  electron  has  not  been 
directly  measured,  there  is  every  reason  to  believe  that  it  is 
identical  with  the  charge  carried  by  the  negative  ion  in  gases. 
The  charge  on  an  electron  is  supposed  to  be  the  smallest 
unit  of  electricity  that  takes  part  in  the  transfer  of  an  elec- 
tric current,  whether  in  solids,  liquids,  or  gases.  There  is  one 
marked  point  of  distinction  between  a  positive  ion  and  an  elec- 
tron. The  electron  in  motion  has  an  apparent  mass  of  about 
1/1000  that  of  the  hydrogen  atom,  while  the  corresponding 
positive  charge  has  never  been  found  associated  with  a  mass  less 
than  that  of  the  hydrogen  atom.  This  has  led  to  the  view  that 
there  is  only  one  kind  of  electricity,  viz.,  negative,  which  is 
associated  with  the  electron,  and  that  a  positively  charged  body 
or  ion  is  one  which  has  been  deprived  of  one  or  more  of  its 
normal  complement  of  electrons. 

RADIATION  FROM  AN  ELECTRON 

An  electron  in  motion  produces  a  magnetic  field  whose  in- 
tensity at  any  point,  for  velocities  small  compared  with  that  of 
light,  is  proportional  to  its  velocity.  This  magnetic  field  travels 
with  the  electron,  and  magnetic  energy  is  consequently  stored 
up  in  the  medium  surrounding  it.  The  amount  of  this  mag- 
netic energy  is  proportional  to  the  square  of  the  velocity  u  of 
the  electron,  and  can  consequently  be  expressed  in  the  form 
\  m  u2.  In  this  equation  m  represents  the  apparent  or  electrical 

2  e2 
mass  of  the  electron,  and  is  equal  to  „ — ,  where  e  is  the  charge 

and  a  the  radius  of  the  electron. 

An  electron  moving  uniformly  in  a  straight  line  does  not 


262  RADIOACTIVE   TRANSFORMATIONS 

radiate  energy,  but  any  change  in  its  motion  is  accompanied  by 
the  dissipation  of  energy  in  the  form  of  electromagnetic  radia- 
tion, which  travels  out  from  the  electron  with  the  velocity  of 
light.  The  rate  of  dissipation  of  radiant  energy  is  proportional 
to  the  square  of  its  acceleration,  and  consequently  becomes 
large  if  an  electron  is  suddenly  set  in  motion  or  brought  to  rest. 
For  example,  the  X-rays  produced  in  a  vacuum  tube  are  be- 
lieved to  consist  of  the  intense  electromagnetic  pulses  which 
are  set  up  by  the  sudden  arrest  of  the  cathode  rays  when  they 
impinge  on  the  anticathode. 

An  electron  constrained  to  move  in  a  circular  orbit  is  a  power- 
ful radiator  of  energy,  since  its  motion  is  always  accelerated 
toward  its  centre.  This  necessary  loss  of  energy  from  an  accel- 
erated electron  has  been  one  of  the  greatest  difficulties  met 
with  in  endeavoring  to  deduce  the  constitution  of  a  stable  atom. 
For  the  supposition  that  an  atom  consists  of  a  number  of  posi- 
itively  and  negatively  charged  particles  in  motion,  held  in 
equilibrium  by  their  mutual  forces,  Larmor1  has  shown  that 
the  condition  for  no  loss  of  energy  by  radiation  is  that  the  vector 
sum  of  the  accelerations  of  all  the  component  charged  particles 
shall  be  permanently  zero.  If  this  condition  is  not  fulfilled 
there  will  be  a  steady  drain  of  internal  energy  from  the  atom  in 
the  form  of  electromagnetic  radiation,  and  unless  this  is  balanced 
in  some  way  by  the  absorption  of  energy  from  outside,  the 
atom  must  ultimately  become  unstable  and  break  up  into  a  new 
system. 

There  are  thus  two  essential  conditions  which  must  be  ful- 
filled for  atoms  to  be  permanent.  The  positively  and  negatively 
charged  particles  constituting  the  atom  must  be  so  arranged  as 
to  form  a  stable  aggregate  under  their  forces  of  attraction  and 
repulsion,  and  at  the  same  time  their  arrangement  and  motions 
must  be  such  that  no  energy  is  radiated  from  the  atom. 

Since  there  is  reason  to  believe  that  the  atoms  of  many  ele- 
ments are  either  permanently  stable  or  else  remain  stable  for 
intervals  of  time  measured  by  millions  of  years,  it  would  appear 
that  these  two  conditions  must  be  very  approximately  realized  in 

1  Larmor  :  Aether  and  Matter,  p.  233. 


RADIOACTIVE   PROCESSES  263 

the  constitution  of  many  of  the  atoms  of  the  different  elements. 
Any  atom  in  which  these  conditions  were  not  fulfilled  must 
long  ago  have  disappeared  and  been  broken  up  into  more  stable 
atomic  systems. 

It  is  thus  not  so  much  a  matter  of  surprise  that  some  atoms 
spontaneously  break  up,  as  that  the  atoms  are  such  stable 
arrangements  as  they  appear  to  be.  The  possibility  of  the 
disintegration  of  the  atom  is  thus  in  most  cases  a  necessary 
consequence  of  modern  theories  of  atomic  constitution. 

REPRESENTATIONS  OF  ATOMIC  CONSTITUTION 

The  recent  developments  in  physical  science  have  given  a 
great  impetus  to  the  study  of  the  constitution  of  the  atom,  and 
attempts  have  been  made  to  form  a  mechanical,  or  rather  elec- 
trical, representation  of  an  atom  which  shall  imitate  as  closely  as 
possible  the  behavior  of  the  actual  atom. 

On  the  electronic  theory  of  matter  it  is  supposed  that  the 
hydrogen  atom  consists  of  about  a  thousand  electrons  held  in 
equilibrium  by  the  internal  forces  of  the  atom.  Since  an  atom 
is  electrically  neutral  in  regard  to  external  bodies,  it  is  necessary 
to  assume  that  the  negative  charge  carried  by  the  electrons  is 
compensated  by  the  presence  within  the  atom  of  an  equal  posi- 
tive charge.  The  electrons  are  supposed  to  be  the  mobile  parts 
of  the  atom,  while  the  positive  electricity  is  more  or  less  fixed 
in  position. 

The  earliest  representation  of  such  a  model  atom  was  given 
by  Lord  Kelvin.1  A  number  of  electrons  or  negatively  charged 
particles  were  supposed  to  be  arranged  within  a  uniform  sphere 
of  positive  electrification.  The  positive  charge  distributed 
throughout  the  sphere  was  equal  in  magnitude  to  the  corespond- 
ing  negative  charges  carried  by  the  mobile  electrons.  This 
arrangement  was  very  ingenious,  for  it  not  only  fulfilled  the 
condition  that  the  atom  was  electrically  neutral,  but  supplied 
the  necessary  forces  within  the  atom  to  hold  the  electrons  in 
equilibrium. 

i  Lord  Kelvin :  Phil.  Mag.,  March,  1903;  Oct.,  1904 ;  Dec.,  1905. 


264  RADIOACTIVE   TRANSFORMATIONS 

Without  such  constraining  forces  it  is  obvious  -that  the  elec- 
trons would  repel  each  other  and  escape  from  the  atom.  Lord 
Kelvin  showed  that  certain  arrangements  of  the  electrons 
throughout  the  sphere  were  in  stable  equilibrium,  while  others 
were  unstable,  and  a  slight  disturbance  would  lead  either  to 
their  escape  from  the  atom  or  to  their  falling  into  a  more  stable 
configuration.  Lord  Kelvin  has  recently  devised  certain  ar- 
rangements of  the  positively  and  negatively  charged  particles 
constituting  the  atom  which  are  unstable,  and  must  lead  to  the 
expulsion  of  either  a  positively  or  negatively  charged  particle 
with  great  velocity,  thus  imitating  the  behavior  of  a  radioatom 
in  expelling  a  and  /?  particles. 

The  conception  of  an  atom,  suggested  by  Lord  Kelvin,  was 
further  developed  by  J.  J.  Thomson.1  A  number  of  electrons 
arranged  in  a  ring  at  definite  angular  intervals  were  supposed 
to  rotate  uniformly  within  a  sphere  of  positive  electrification. 
He  drew  attention  to  a  remarkable  property  of  such  a  config- 
uration. We  have  seen  that  a  single  electron  moving  in  a 
circular  orbit  radiates  energy,  and  the  amount  of  the  radia- 
tion becomes  large  when  the  electron  is  supposed  to  describe 
a  circular  orbit  of  atomic  dimensions.  When,  however,  a  num- 
ber of  electrons  followed  one  another  in  a  circle,  the  fraction 
of  the  energy  of  motion  of  the  electrons  radiated  per  revolu- 
tion decreased  very  tepidly  as  the  number  of  electrons  in  the 
ring  increased. 

For  example,  the  radiation  from  a  group  of  six  electrons  mov- 
ing with  a  velocity  of  one  tenth  that  of  light  is  less  than  one 
millionth  of  that  of  a  single  particle.  For  a  velocity  of  one 
hundredth  that  of  light  the  amount  of  radiation  is  only  10~16  of 
that  from  a  single  electron  moving  with  the  same  velocity  in 
the  same  orbit. 

Such  results  show  that  an  atom  consisting  of  a  number  of 
rotating  electrons  may  radiate  energy  extremely  slowly,  but 
ultimately  this  slow  continuous  drain  of  energy  from  the  atom 
results  in  a  diminution  of  velocity  of  the  electrons.  When  this 
velocity  falls  below  a  certain  critical  value,  the  atom  becomes 

1  J.  J.  Thomson  :  Phil.  Mag.,  Dec.,  1903 ;   March,  1904. 


RADIOACTIVE  PROCESSES  265 

unstable,  and  either  breaks  up  with  the  expulsion  of  a  part  of 
the  atom,  or  forms  a  new  arrangement  of  the  electrons. 

J.  J.  Thomson  considers  that  the  cause  of  the  disintegration 
of  the  atoms  of  radioactive  matter  must  be  ascribed  to  the  loss 
of  energy  by  radiation  from  the  atom.  He  has  mathematically 
investigated  the  possible  temporarily  stable  arrangements  of  a 
given  number  of  electrons  within  a  sphere  of  uniform  positive 
electrification.  The  properties  of  such  an  atom  are  very  strik- 
ing, and  indirectly  suggest  a  possible  explanation  of  the  periodic 
law  of  arrangement  of  the  elements  in  chemistry.  When  the 
electrons  revolve  in  one  plane  they  tend  to  arrange  themselves 
in  a  number  of  concentric  rings,  and  generally,  if  free  to  move 
in  any  plane,  in  a  number  of  concentric  shells,  like  the  coats  of 
an  onion. 

It  is  not  necessary  here  to  consider  in  detail  the  arrangements 
discussed  by  J.  J.  Thomson,  for  these  have  already  been  given 
by  him  in  the  Silliman  Lectures  of  two  years  ago.  It  suffices 
to  say  that  such  a  model  atom  imitates  in  a  remarkable  way 
the  behavior  of  the  atoms  of  the  elements,  and  also  suggests  a 
possible  explanation  of  valency. 

Some  configurations  of  electrons,  for  example,  are  able  to  lose 
one,  others  two  or  more  electrons,  and  yet  remain  stable.  Others 
are  able  to  gain  an  additional  electron  6r  two  without  altering 
the  main  features  of  the  arrangement  of  the  electrons.  Those 
atoms  which  can  readily  lose  electrons  would  correspond  to  an 
electropositive  element,  and  vice  versa. 

Such  attempts  to  imitate  by  an  electrical  model  the  structure 
of  the  atom  are  of  necessity  somewhat  artificial,  but  they  are  of 
great  value  as  indicating  the  general  method  of  attack  of  the 
greatest  problem  that  at  present  confronts  the  physicist.  As 
our  knowledge  of  atomic  properties  increases  in  accuracy  it  may 
yet  be  possible  to  deduce  a  structure  of  the  atom  which  fulfils 
the  conditions  required  by  experiment.  A  promising  beginning 
has  already  been  made,  and  there  is  every  hope  that  still  further 
advances  will  soon  be  made  in  the  elucidation  of  the  mystery  of 
atomic  structure. 

We  have  seen  that  on  present  theories  positive  electricity 


266  RADIOACTIVE   TRANSFORMATIONS 

plays  a  very  different  role  from  negative  electricity.  In  order 
to  hold  the  electrons  together  and  to  make  the  atom  electrically 
neutral,  it  is  necessary  to  call  in  the  assistance  of  a  fixed  dis- 
tribution of  positive  electrification.  The  mobile  electrons  con- 
stitute, so  to  speak,  the  bricks  of  the  atomic  structure,  while  the 
positive  electricity  acts  as  the  necessary  mortar  to  bind  them 
together.  This  appears  to  be  a  somewhat  arbitrary  arrangement, 
but  at  present  there  appears  to  be  no  escape  from  this  funda- 
mental difficulty  of  the  difference  between  positive  and  negative 
electricity. 

CAUSES  OF  ATOMIC  DISINTEGRATION 

We  are  now  in  a  position  to  consider  the  possible  causes  that 
lead  to  the  disintegration  of  the  atoms  of  the  radioelements.  It 
has  been  shown  that  the  law  controlling  the  rate  of  disintegra- 
tion of  any  individual  product  is  very  simple.  The  number  of 
atoms  breaking  up  per  second  is  always  in  a  constant  ratio  to 
the  total  number  present.  The  value  of  this  ratio,  however, 
varies  enormously  for  the  different  products.  It  has  not  been 
found  possible  to  alter  the  rate  of  disintegration  of  any  product 
by  any  external  agency.  Difference  of  temperature,  which  plays 
such  an  important  part  in  altering  the  rate  of  chemical  reac- 
tions, is  entirely  without  influence  on  the  rate  of  transformation 
of  the  radioactive  bodies.  For  example,  the  heat  emission  of 
radium,  which  is  a  measure  of  the  kinetic  energy  of  the  a  par- 
ticles, is  unaltered  by  plunging  the  radium  into  liquid  hydrogen. 
Elevation  of  temperature  or  chemical  actions  are  equally  without 
influence. 

It  thus  appears  that  the  atoms  of  the  radioelements  suffer 
spontaneous  disintegration,  or  that  it  is  brought  about  by  forces 
beyond  our  control.  It  has  been  suggested  that  the  atoms  of 
radioactive  matter  may  act  as  transformers  of  energy,  abstracted 
in  some  way  from  the  surrounding  medium.  Theories  of  this 
character  were  put  forward  for  the  special  purpose  of  account- 
ing for  the  emission  of  heat  from  radium  without  regard  to  the 
nature  of  the  other  radioactive  processes.  There  is  indubutable 
evidence  that  the  heating  effect  of  radium  is  a  necessary  conse- 


RADIOACTIVE   PROCESSES  267 

quence  of  the  transformation  of  the  radioatoms,  and  is  a  second- 
ary effect  resulting  from  the  energy  of  motion  of  the  expelled 
a  particles. 

Such  theories  do  not  take  into  account  the  fact  that  radioac- 
tivity is  always  accompanied  by  the  appearance  of  new  types  of 
active  matter.  There  must  consequently  be  chemical  changes 
in  the  active  matter  and,  from  other  data,  it  is  concluded  that 
the  changes  occur  in  the  atom  itself  and  not  in  the  molecule. 

The  causes  which  lead  to  the  disintegration  of  the  atom  are  at 
present  a  matter  of  conjecture.  It  is  not  yet  possible  to  decide 
with  certainty  whether  the  disintegration  is  due  to  an  external 
cause,  or  is  an  inherent  property  of  the  atom  itself.  It  is  con- 
ceivable, for  example,  that  some  unknown  external  force  may 
supply  the  necessary  disturbance  to  cause  disintegration.  In 
such  a  case,  the  external  force  supplies  the  place  of  a  detonator 
to  precipitate  the  atomic  explosion.  The  energy  liberated  by  the 
explosion,  however,  is  derived  mainly  from  the  atom  itself  and 
not  from  the  detonator.  The  law  of  transformation  of  radioactive 
matter  does  not  throw  any  light  on  the  question,  for  such  a  law 
is  to  be  expected  on  either  hypothesis. 

It  seems,  however,  most  probable  that  the  primary  cause  of 
atomic  disintegration  must  be  looked  for  in  the  atom  itself,  and 
consists  in  the  loss  of  energy  from  the  atom  in  the  form  of 
electromagnetic  radiation.  We  have  seen  that  unless  certain 
conditions  are  fulfilled,  an  atom  composed  of  negatively  and 
positively  charged  particles  will  lose  energy  by  radiation  and 
ultimately  break  up. 

For  example,  we  have  seen  that  J.  J.  Thomson  has  devised 
certain  models  of  atoms  which  radiate  energy  extremely  slowly, 
but  which  must  ultimately,  in  consequence  of  the  loss  of  atomic 
energy,  become  unstable  and  either  break  up  or  form  a  new 
atomic  system.  In  the  case  of  primary  elements  like  uranium 
and  thorium,  the  atoms  are  comparatively  stable  and  have  an 
average  life  of  a  thousand  million  years.  The  question  then 
arises  whether  this  radiation  of  energy  is  continuously  occur- 
ring in  all  the  atoms,  or  whether  only  a  minute  fraction  is 
involved  at  one  time.  On  the  first  view,  all  atoms  formed  at 


268  RADIOACTIVE   TRANSFORMATIONS 

the  same  time  should  last  for  a  definite  interval.  This,  how- 
ever, is  contrary  to  the  observed  law  of  transformation,  in  which 
the  atoms  theoretically  have  a  life  embracing  all  values  from 
zero  to  infinity. 

We  thus  arrive  at  the  conclusion  that  the  configuration  of 
the  atom  which  gives  rise  to  a  radiation  of  energy  only  occurs 
in  a  minute  fraction  of  the  atoms  present  at  one  time,  and  is 
probably  governed  purely  by  the  laws  of  probability. 

There  is  one  peculiarity  of  the  transformation  of  the  products 
of  uranium,  thorium,  radium,  and  actinium  which  is  possibly 
important  in  this  connection.  The  ft  rays  appear  only  in  the 
last  of  the  rapid  series  of  changes  of  these  elements,  and  these 
are  expelled  with  enormous  velocity.  After  the  expulsion  the 
resulting  product  is  either  permanently  stable  or  far  more  stable 
than  the  preceding  ones.  It  would  appear  more  than  a  coin- 
cidence that  the  expulsion  of  a  high  velocity  /3  particle  should 
occur  only  in  this  final  stage  for  each  element.  It  is  possible 
that  the  0  particle,  which  is  finally  expelled,  is  the  active  agent 
in  promoting  the  previous  transformations,  and  that  when  once 
the  disturbing  factor  has  been  removed,  the  resulting  atom  sinks 
into  a  configuration  of  far  more  stable  equilibrium. 

For  example,  one  of  the  electrons  composing  the  atom  may 
take  up  a  position  in  the  atomic  system  which  leads  to  a 
radiation  of  energy.  As  a  result  the  atom  breaks  up  with  the 
expulsion  of  an  a  particle,  and  this  process  continues  through 
successive  stages  until,  finally,  there  is  a  violent  explosion 
within  the  atom,  which  results  in  the  expulson  of  the  disturbing 
electron  with  enormous  speed. 

PROCESSES  OCCURRING  IN  RADIUM 

Consider  a  minute  quantity  of  radium  of  weight  about  one 
millionth  of  a  milligram  in  radioactive  equilibrium.  This  will 
contain  about  3.6  x  1012  atoms  of  radium  of  atomic  weight  225. 
Since  in  one  gram  of  radium  6.2  x  1010  a  particles  are  expelled 
per  second  from  the  radium  itself,  the  number  disintegrating  per 
second  in  one  millionth  of  a  milligram  is  62.  On  an  average, 
an  exactly  equal  number  of  a  particles  will  be  expelled  from 


RADIOACTIVE   PROCESSES  269 

each  of  the  successive  a  ray  products,  viz.,  the  emanation, 
radium  A,  and  radium  C. 

The  number  of  atoms  of  each  of  the  radioactive  products 
present  with  this  quantity  of  radium  will  be  very  different. 
For  3.6  x  1012  atoms  of  radium,  there  will  be  about  3  x  107 
of  atoms  of  emanation,  1.6  x  104  atoms  of  radium  A,  1.5  x  105 
of  radium  B,  and  1.15  x  105  of  radium  C.  The  radium  atoms 
present  will  thus  enormously  preponderate  over  those  of  its 
products. 

Supposing  it  were  possible  to  magnify  this  small  particle  of 
radium  so  as  to  distinguish  the  individual  atoms,  we  should  see 
a  large  number  of  radium  atoms,  and  mixed  with  them  a  very 
small  number  of  atoms  of  its  products ;  but  if  the  attention  were 
focussed  on  the  atoms  of  each  of  the  individual  substances  pres- 
ent we  should  find  that  the  same  number  of  a  particles  are  ex- 
pelled from  each  of  them  per  second.  The  number  of  atoms  of 
each  product  remains  on  an  average  constant,  for  the  supply  of 
fresh  atoms  compensates  for  those  that  break  up. 

The  electronic  theory  of  matter  supposes  that  an  atom  is 
composed  of  a  swarm  of  electrons  in  rapid  movement  held  in 
equilibrium  by  the  internal  forces  of  the  atom.  In  the  case  of 
heavy  atoms,  like  those  of  the  radioelements,  it  is  not  necessary 
to  suppose  that  each  of  these  electrons  has  complete  freedom  of 
movement.  The  character  of  the  transformation  of  the  atom 
suggests  that  it  is  built  up  in  part  of  a  number  of  secondary 
units,  consisting  of  groups  or  aggregates  of  electrons  in  equilib- 
rium, which  are  in  rapid  independent  motion  within  the  atom. 

For  example,  it  seems  probable  that  the  a  particle  or  helium 
atom  actually  exists  as  an  independent  unit  of  matter  within 
the  radium  atom,  and  is  released  at  the  moment  of  the  disin- 
tegration of  the  latter.  These  a  particles"are  in  rapid  movement, 
and  when  a  stage  of  instability  is  reached,  one  is  ejected  from 
the  atom  with  the  velocity  it  possessed  in  its  atomic  orbit.  If 
this  be  the  case,  the  a  particles,  on  an  average,  must  have  a 
velocity  within  the  atom  of  more  than  1/30  that  of  light. 

It  is  possible,  however,  that  a  part  of  their  great  kinetic 
energy  may  be  acquired^  during  the  process  of  expulsion  from 


270  RADIOACTIVE   TRANSFORMATIONS 

the  atom,  for  we  have  seen  that,  from  a  variety  of  considera- 
tions, the  atom  must  be  supposed  to  be  the  seat  of  intense  elec- 
trical forces.  At  the  moment  preceding  the  expulsion  of  an  a 
particle,  from  radium  for  example,  the  atom  must  be  in  a  state 
of  violent  disturbance.  As  a  result,  the  forces  which  constrain 
one  of  these  a  particles  within  the  atom  are  momentarily  neu- 
tralized and  the  a  particle  escapes  from  the  atom  at  an  enormous 
speed. 

The  internal  forces  are  still  sufficiently  powerful  to  prevent 
the  escape  of  the  other  parts  of  the  atom,  and  there  is  a  rapid 
adjustment  of  its  components  to  form  a  new  system  which  is 
temporarily  stable.  It  is  probable  that  for  a  short  interval 
after  the  escape  of  the  a  particle,  the  atom  is  in  a  state  of 
violent  disturbance,  but  finally  sinks  again  into  a  temporarily 
stable  system.  The  residual  atom  has  smaller  mass  than  before, 
and  the  internal  arrangement  of  its  parts  is  entirely  different 
from  the  previous  one.  The  new  atom,  in  fact,  becomes  an 
atom  of  the  emanation,  and  has  chemical  and  physical  properties 
entirely  different  from  those  of  the  parent  atom. 

The  atoms  of  the  emanation  are  not  nearly  so  stable  as  those 
of  radium,  for  they  have  an  average  life  of  only  six  days.  The 
atom  breaks  up  as  before,  expelling  another  a  particle  and  giv- 
ing rise  to  an  atom  of  radium  A,  which  again  differs  widely  in 
properties  from  those  of  the  emanation  and  of  radium.  This 
substance  is  very  unstable,  for  the  atoms  have  an  average  life  of 
only  four  minutes.  After  the  loss  of  another  a  particle  the 
atom  of  radium  B  makes  its  appearance.  This  atom  undergoes 
a  change,  which  is  apparently  different  in  character  from  the 
others.  It  may  or  may  not  expel  an  a  particle,  but  if  it  does 
so,  the  particle  travels  at  too  low  a  speed  to  produce  appre- 
ciable ionization  in  the  gas.  The  atom,  however,  expels  a  /3  par- 
ticle at  a  moderate  speed.  The  atom  of  radium  B  then  changes 
into  C.  The  instability  of  the  latter  atom  results  in  an  explo- 
sion of  great  intensity.  An  a  particle  is  expelled  at  a  greater 
speed  than  from  any  other  product,  while  at  the  same  time  a  /? 
particle  is  ejected  with  a  velocity  nearly  equal  to  that  of  light. 
After  this  violent  explosion  the  resulting  atom  sinks  into  a  far 


RADIOACTIVE   PROCESSES  271 

more  stable  system,  but  this  eventually  breaks  up,  as  we  have 
seen,  and  passes  through  still  further  stages.  Finally,  after  the 
expulsion  of  an  a  particle  from  radium  F,  the  resulting  atom  is 
probably  identical  with  that  of  lead. 

Such  considerations  show  that  a  mass  of  radium  is  the  seat  of 
an  extraordinary  conflict  of  forces.  In  a  gram  of  radium,  for 
example,  about  6  x  1010  a  particles  are  shot  out  each  second 
from  each  of  the  a  ray  products,  while  in  addition  there  is  an 
expulsion  of  an  equal  number  of  high  velocity  electrons  from 
radium  B  and  C.  Since  the  a  particles  are  only  able  to  pass 
through  a  small  thickness  of  matter,  the  greater  number  of 
these  a  particles  are  stopped  in  the  radium  itself,  which  is  con- 
sequently exposed  to  a  bombardment  of  great  intensity. 

Let  us  concentrate  our  attention  for  a  moment  on  an  atom  of 
radium  at  the  moment  of  the  expulsion  of  an  a  particle.  If  the 
principle  of  equivalence  of  momenta  holds,  the  expulsion  of  the 
a  particle  must  cause  a  recoil  of  the  atom  from  which  it  escapes. 
Since  the  a  particle  has  a  mass  of  about  1/50  that  of  the  radium 
atom,  and  is  expelled  with  a  velocity  of  nearly  2  x  109  cms.  per 
second,  the  atom  of  radium  must  recoil  with  an  initial  velocity 
of  about  4  x  107  cms.  per  second,  or  about  200  miles  per  sec- 
ond. This  velocity  will  decrease  rapidly  in  consequence  of  the 
collisions  of  the  moving  atom  with  the  atoms  in  its  path,  and  it 
will  probably  be  brought  nearly  to  rest  after  traversing  a  very 
small  distance.  The  kinetic  energy  of  motion  of  the  radium 
atom  will  thus  be  transformed  into  heat.  The  a  particle  in- 
itially starts  with  an  enormous  velocity,  and  must  force  its  way 
through  the  atoms  of  radium  in  its  path,  knocking  off  from 
them  a  shower  of  electrons  in  the  process.  Its  energy  is  gradu- 
ally used  up  in  producing  ions,  and  its  velocity  consequently 
diminishes.  Finally,  it  loses  "its  power  of  ionizing,  and  is 
brought  to  rest.  Its  charge  is  neutralized,  and  the  a  particle 
then  becomes  a  helium  atom,  and  is  mechanically  imprisoned  in 
the  mass  of  the  radium.  The  energy  used  up  in  producing  ions 
in  the  radium  is  finally  given  up  in  the  form  of  heat,  for  the 
ions  recombine,  emitting  heat  during  the  process. 

Let.  us  now  turn  our  attention  to  the  processes  occurring  in 


272  EADIOACTIVE   TRANSFORMATIONS 

the  air  or  other  gas  surrounding  the  radium.  The  a  particles 
expelled  from  the  surface  of  a  mass  of  radium  escape  into  the 
air  without  loss  of  speed  due  to  traversing  the  radium,  and  the 
a  particle  from  each  product  has  its  characteristic  velocity. 
Imagine  that  we  can  follow  visually  the  flight  of  an  a  particle 
through  the  gas.  The  velocity  of  the  a  particle  is  initially  so 
great  compared  with  the  velocity  of  translation  of  the  molecules 
of  the  gas,  that  the  latter  will  appear  to  stand  still  during  the 
flight  of  the  a  particle.  The  time  is  too  short  for  the  molecules 
of  air  to  escape  from  the  path  of  the  a  projectile,  and  its  veloc- 
ity and  energy  are  so  great  that  it  is  able  to  plunge  through  the 
molecules  in  its  path.  The  electric  disturbance  produced  by  its 
passage  through  the  molecule  may  lead  to  the  expulsion  of  an 
electron,  and,  probably  in  many  cases,  to  the  breaking  up  of  the 
complex  molecule  into  charged  atoms. 

Two  or  more  ions  are  consequently  produced  as  a  result  of 
the  passage  through  the  molecule.  This  process  continues  until, 
after  passing  through  about  3.5  cms.  of  air  under  normal  condi- 
tions, and  producing  about  100,000  ions,  its  power  of  ionization 
is  lost.  Exactly  what  happens  to  the  a  particle  at  the  end  of  its 
career  of  ionization  is  not  yet  known.  As  we  have  seen,  experi- 
ment indicates  that  some  of  the  a  particles  are  still  moving  at  a 
high  velocity  when  their  ionizing  power  becomes  very  small. 
Since  the  initial  rapid  reduction  of  the  velocity  of  the  a  particle 
appears  to  be  mainly  a  result  of  the  energy  used  up  in  ionizing 
the  gas,  it  is  probable  that  the  a  particle,  after  its  power  of  ioniz- 
ing has  been  largely  lost,  will  traverse  a  considerable  distance  of 
air  before  it  is  brought  to  rest  by  continued  collision  with  the 
gas  molecules.  We  are  unable  to  detect  the  presence  of  such  a 
particles,  since  they  have  lost  all  the  properties  which  serve 
ordinarily  to  detect  their  presence. 

There  is  a  considerable  amount  of  evidence  to  show  that  the 
energy  absorbed  from  the  a  particle  in  producing  a  pair  of  ions 
is  much  greater  than  that  required  merely  to  separate  the  posi- 
tive ion  from  the  negative.  Such  a  result  suggests  that  during 
the  process  of  ionization  the  ions  acquire  a  considerable  velocity, 
and  that  the  energy  spent  in  setting  the  ions  in  motion  is  large 


RADIOACTIVE   PROCESSES  273 

compared  with  that  necessary  for  the  mere  separation  of  the 
ions  from  the  immediate  sphere  of  each  other's  influence. 

Although  the  ft  particle  escapes  -from  the  radium  with  an 
average  velocity  ten  times  that  of  the  .a  particle,  it  is  far  less 
efficient  as  an  ionizer.  It  produces  only  a.  small  number  of  ions 
per  centimeter  of  path  compared  with  the  number  produced  by 
an  a  particle,  and  passes  through  about  100  times  the  distance  in 
air  before  it  ceases  to  ionize. 

We  have  not  so  far  considered  the  connection  of  the  7  rays 
with  radioactive  changes.  These  rays  always  appear  with  the  0 
rays  and  are  believed  to  be  an  electromagnetic  pulse,  set  up  in 
consequence  of  the  sudden  expulsion  of  the  ft  particle.  This 
pulse  is  the  seat  of  very  intense  electric  and  magnetic  forces 
and  travels  out  from  the  atom  like  a  spherical  wave  with  the 
velocity  of  light.  Such  a  pulse  is  a  very  inefficient  ionizer  com- 
pared with  the  a  particle,  and  on  an  average  produces  only  one 
ion  per  centimeter  of  its  path  for  10,000  produced  by  the  a  par- 
ticle. The  penetrating  power  of  the  7  ray,  on  the  other  hand,  is 
very  great,  and  it  continues  to  produce  ions  even  after  travers- 
ing a  great  distance  of  air. 

The  energy  of  the  rays  which  traverse  the  gas  is  ultimately 
frittered  down  into  heat.  The  initial  energy  of  motion  of  the 
ions  is  rapidly  lost  by  collision  with  the  gas  particles,  while  the 
ions  finally  recombine  with  the  liberation  of  energy. 

In  addition  to  the  ionization  effects  already  considered,  there 
are  very  marked  secondary  effects  produced  when  the  rays 
impinge  upon  matter.  The  a  particles  release  a  shower  of 
electrons  from  the  matter  upon  which  they  impinge.  These 
electrons,  however,  are  emitted  at  a  very  slow  speed.  On  the 
other  hand,  the  ft  and  7  rays  cause  the  release  of  electrons  at  a 
speed  comparable  with  that  of  light.  These  secondary  radia- 
tions are  most  marked  when  the  radiations  fall  on  heavy  metals 
like  lead,  but  no  doubt  occur,  though  in  a  much  less  intense 
degree,  during  the  passage  of  the  rays  through  a  gas. 

On  account  of  the  great  energy  of  motion  of  the  a  particle,  it 
might  be  expected  to  set  in  vibration  the  atoms  of  matter  in  its 
path,  and  cause  them  to  emit  light  waves.  This  property  of  the 

18 


274  RADIOACTIVE   TRANSFORMATIONS 

a  rays  of  exciting  luminosity  was  first  noted  by  Sir  William  and 
Lady  Huggins,1  who  found  that  the  weak  phosphorescent  light 
of  radium  showed  the  band  spectrum  of  nitrogen.  This  has 
been  traced  to  the  action  of  the  a  rays,  either  in  free  nitrogen 
close  to  the  radium,  or  in  nitrogen  occluded  within  the  radium 
compound.  Such  a  result  is  of  unusual  interest,  as  it  is  the 
first  example  of  a  gas  giving  a  spectrum  when  cold  without  the 
stimulus  of  a  strong  electric  discharge. 

Walter  and  Pohl 2  have  recently  found  that  the  gas  traversed 
by  the  rays  from  an  active  preparation  of  radiotellurium  emits 
light  waves  which  act  on  a  photographic  plate.  The  intensity 
of  this  effect  is  greatest  for  pure  nitrogen.  The  atoms  of  nitro- 
gen appear  to  be  more  easily  stimulated  to  give  out  their  char- 
acteristic vibrations  than  any  other  gas  so  far  examined.  It  is 
a  matter  of  surprise  that  as  yet  no  evidence  has  been  obtained 
that  the  a  particles  themselves  give  a  spectrum.  The  violent 
collisions  of  the  a  particle  with  the  molecules  in  its  path  must 
set  up  vibrations  in  the  a  particle,  and  it  should  give  a  charac- 
teristic spectrum.  Experiments  of  this  character,  though  of 
great  difficulty,  are  most  important,  for  they  may  throw  light  on 
the  nature  of  the  a  particle.  In  this  connection  it  is  of  interest 
to  note  that  Giesel  found  that  a  preparation  of  "emanium" 
emitted  a  phosphorescent  light  consisting  of  bright  lines.  These 
lines  were  found  to  be  due  to  didymium,  which  was  present  as 
an  impurity  in  the  active  substance. 

There  is  no  doubt  that  the  radiations  emitted  from  active 
bodies  serve  as  very  powerful  agents  for  the  ionization  and  dis- 
sociation of  matter.  No  definite  evidence  has  so  far  been  ob- 
tained that  the  a  or  /3  particles  emitted  from  radium  are  able  to 
hasten  its  rate  of  transformation.  Such  a  concentrated  source 
of  energy  as  these  high  velocity  particles  might  be  expected  to 
produce,  under  some  conditions,  a  disintegration  of  the  atoms  of 
matter  through  which  they  pass.  A  mass  of  radium,  for  exam- 
ple, which  is  subjected  to  an  intense  bombardment  by  its  own  a 

1  Sir  William  and  Lady  Huggins:  Proc.  Roy.  Soc.,  Ixxii,  pp.  196,  409  (1903) ; 
Ixxvii,  p.  130  (1906). 

2  Walter  and  Pohl:  Ann.  d.  Phys.,  xviii,  p.  406  (1905). 


RADIOACTIVE  PROCESSES  275 

and  /3  particles  might  be  expected  to  disintegrate  faster  than  the 
same  amount  of  radium  diffused  through  a  large  volume.  Fur- 
ther experiments  in  this  direction  may  yet  show  that  such  an 
effect  does  exist,  but  it  is  certainly  not  very  marked.  A  direct 
attack  on  the  question  as  to  whether  X-rays  are  able  to  cause 
the  disintegration  of  matter  has  been  made  by  Bumstead.1  A 
strong  beam  of  X-rays  fell  on  two  plates  of  zinc  and  lead, 
which  were  of  such  a  thickness  as  to  absorb  an  equal  fraction  of 
energy  of  the  incident  beam.  The  lead  was  raised  to  a  consider- 
ably higher  temperature  than  the  zinc,  indicating  that  although 
the  same  fraction  of  the  energy  of  the  rays  had  been  absorbed, 
more  energy  had  been  released  in  the  lead  than  in  the  zinc. 
Such  a  result  suggests  that  the  X-rays  cause  a  greater  amount 
of  atomic  disintegration  in  lead  than  in  zinc,  and  that  a  consider- 
able portion  of  the  heat  generated  in  the  lead  is  due  to  the 
energy  liberated  from  the  transformation  of  its  atoms. 

Further  experiments  with  a  variety  of  metals  and  with  differ- 
ent sources  of  intense  ionizing  radiations  are  required  to  sub- 
stantiate completely  such  a  far-reaching  conclusion,  but  the 
results  so  far  obtained  in  this  difficult  field  of  research  certainly 
lead  us  to  hope  that  we  may  yet  bring  about  the  disintegration 
of  atoms  by  laboratory  methods. 

We  have  previously  indicated  that  there  is  a  very  strong 
proof  that  ordinary  matter  possesses  the  property  of  emitting 
characteristic  radiations  which  are  able  to  ionize  a  gas.  Such 
results  suggest  that  there  is  an  extremely  slow  transformation  of 
matter  of  a  type  similar  to  that  shown  by  the  radioactive  bodies. 
It  is  not  necessary  to  suppose  that  the  a  particles  from  all  types 
of  matter  is  the  same  mass.  For  example,  hydrogen  may  be 
expelled  from  some  bodies  instead  of  helium.  The  experimen- 
tal observation  that  the  a  particle  loses  its  power  of  acting  on  a 
photographic  plate  and  of  ionizing  the  gas  when  its  velocity 
falls  to  about  8  x  108  cms.  per  second  is  of  great  importance  in 
this  connection.  There  is  no  doubt  that  if  a  particles  were  ex- 
pelled from  matter  below  this  velocity  they  would  produce  very 
little  if  any  electrical  effect.  It  is  certainly  a  matter  of  remark 

1  Bumstead:  Phil.  Mag.,  Feb.,  1906. 


276  RADIOACTIVE   TRANSFORMATIONS 

that  the  average  a  particle  from  radioactive  bodies  is  projected 
at  a  speed  less  than  twice  this  minimum  velocity.  It  appears 
by  no  means  improbable  that  the  so-called  radioactive  bodies 
may  differ  from  ordinary  matter  mainly  in  their  power  of  ex- 
pelling a  particles  above  this  critical  speed.  Ordinary  matter 
which  produces  extremely  weak  ionizing  effects  might  be  emit- 
ting a  particles  at  a  rate  comparable  with  uranium,  but  yet,  if 
their  power  of  expulsion  was  less  than  this  critical  value,  it 
would  be  difficult  to  detect  their  presence. 

Such  considerations  show  that  it  is  by  no  means  necessary 
to  suppose  that  the  transformation  of  matter  should  always  be 
accompanied  by  the  intense  electrical  and  other  effects  exhibited 
by  the  radioactive  bodies  proper.  Matter  may  be  undergoing 
slow  atomic  transformation  of  a  character  similar  to  radium, 
which  would  be  difficult  to  detect  by  our  present  methods. 


INDEX. 


INDEX 


a  RATS 

discovery  of,  1 1 

production  of  scintillations  by,  12 

nature  of,  20 

connection  of  a   particle   with   helium, 

181  and  184 

properties  of,  219  et  seq. 
velocity  and  mass  of,  220  and  229 
homogeneous  character  of,  from  radium 

C,  220 

magnetic  deflection  of,  223  et  seq. 
retardation     of     velocity     in     passing 

through  matter,  224  et  seq. 
connection  between  photographic  action 

and  velocity,  225 
minimum  velocity  to  produce  ionization, 

225 

phosphorescent  effects  produced  by,  226 
electrostatic  deflection  of,  227 
deflection  of,  from  thorium  B,  229 
scattering  of,  230 
magnetic    deflection   of,  from  a  thick 

layer  of  radium,  232 
absorption  of,  by  matter,  235  et  seq. 
law  of  absorption  of,  236  et  seq. 
connection  between  ionization  and  ab- 
sorption, 236  et  seq. 
ionization  curves  of  radium  and  radium 

C,  239 

range  of,  from  radium,  240 
ionization  curve  of,  for  thick  layer  of 

radium,  243 
charge  carried  by,  245 
heating  effect  of,  247  et  seq. 
action  of,  in  producing  hydrogen  and 

oxygen,  253 

summary  of  properties  of,  254 
causes  of  expulsion  of,  266  et  seq. 
physical  need  of  effects  of,  268  et  seq. 
action  of,  in  producing  luminosity  in 

gases,  274 
Abraham 

electrical  mass,  10 


Absorption 

of  a  rays  by  matter,  235  et  seq. 

connection  of,  with  ionization,  236 

range  in  air  of  a  rays,  239  et  seq. 

connection  between,  and  atomic  weight, 

243 
Actinium 

discovery  of,  9  and  164 

rapid  escape  of  emanation  from,  1GJ> 

radiations  from,  166 

analysis  of  active  deposit  of,  166 

separation  of  actinium  X  from,  167 

separation  of  radioactinium  from,  107 

transformation  products  of,  168 
Actinium  A 

period  and  properties  of,  166 
Actinium  B 

period  and  properties  of,  166 

electrolysis  of,  167 

volatilization  of,  167 
Actinium  X 

separation  of,  167 

period  and  properties  of,  167 
Adams 

radioactivity  of  spring  water,  207 
Age 

of  radium,  148 

of  earth's  internal  heat,  213  et  seq. 

of  radioactive  minerals,  187  et  seq. 
Allan,  S.  J. 

radioactivity  of  snow,  200 
Allan,  S.  J.  and  Rutherford 

radioactivity  of  atmosphere,  199 
Allen,  H.  S.  and  Lord  Blythswood,  201 

radium  emanation  in  hot  springs,  201 
Atmosphere 

radioactivity  of,  196  et  seq. 

excited  activity  from,  198  et  seq. 

presence  of  emanations  of  radium  and 
thorium  in,  199 

diffusion  of  emanations  into,  203 

amount  of  radium  emanation  in,  204  et 
seq. 


280 


INDEX 


penetrating  radiation  present  in,  207 
electrical  state  of,  208  et  seq. 
Atomic  weight 
of  radium,  8 
of  products  of  radium,  1 93 

/8  RATS 

discovery  of,  9 
nature  of,  10 

variation  of  mass  with  velocity,  10 
electroscope  for  measurement  of,  29 
emission  of,  by  products,  169 
connection  of,  with  atomic  explosion,  169 
Barnes  and  Rutherford 

heating  effect  of  radium  emanation,  90 
heating  effect  of  products  of  radium, 

247  et  seq. 
connection  of  a  rays  with  heating  effect, 

247  et  seq. 
Barrell 

production   of  radium    in    interior    of 

earth,  194 
Becquerel,  H. 

discovery  of  radioactivity  of  uranium,  5 
mass  and  velocity  of  the  0  particle,  10 
separation  of  UrX,  162 
magnetic  deflection  of  a  rays,  221 
curvature  of  path  of  a  rays  in  a  mag- 
netic field,  233 
Blanc 

presence     of    thorium    emanation    in 

spring  water,  201 
Blythswood,  Lord,  and  Allen,  II.  S. 

emanation  in  hot  springs,  201 
Bodlander  and  Ruuge 

evolution   of    gases   from    radium    so- 
lutions, 252 
Boltwood 

amount  of  radium  in  minerals,  152  et 

seq. 

production  of  radium  by  uranium,  158 
lead  as  final  product  of  radium,  191 
products    of    transformation  of    radio- 
elements,  192 
Boltwood  and  Rutherford 

amount  of  radium  in  minerals,  156 
position    of    actinium     in    radioactive 

series,  177 
Bragg 

scattering  of  a  and  )8  rays,  230 
magnetic  deflection  of  a  rays,  233 
absorption  of  a  rays,  233  et  seq. 


Bragg  and  Kleeman 

absorption  of  a  rays,  236  et  seq. 

iouization  curves  of  rays  from  radium 

240  et  seq. 
Bronson 

direct  deflection  electrometer,  33 

effect  of  temperature  on  active  deposit 

of  radium,  118 
Brooks,  Miss 

volatility  of  radium  B,  115 

decay    curves    of    excited    activity    of 

actinium,  166 
Brooks  and  Rutherford 

diffusion  of  radium  emanation,  82 
Bumstead 

presence     of    thorium    emanation     iu 
atmosphere,  199 

disintegration  of  atoms  by  X-rays,  275 
Bumstead  and  Wheeler 

period  of  radium  emanation,  72 

diffusion  of  radium  emanation,  82 

radium  emanation  in  atmosphere,  199 
Burton  and  McLennan 

radium  emanation  in  petroleum,  202 

CAMPBELL 

radioactivity  of  ordinary  matter,  217 
Changes.     (See  Transformations) 
Charge 

carried  by  the  ions,  3 

carried  by  the  o  particle,  184  et  seq. 

carried  by  the  a  rays,  245 
Collie  and  Ramsay 

Spectrum  of  emanation,  89 
Collision 

ionization  of  a  particle  by,  184 

number  of  ions  produced  by,  271 
Concentration 

of    excited    activity    on    the    negative 

electrode,  45 
Condensation 

of  water  round  the  ions,  3 

of  radium  and  thorium  emanations,  76 

et  seq. 
Conductivity 

of  air  in  closed  vessels,  197 
Conservation  of  radioactivity 

examples  of,  64 
Cooke,  H.  L. 

penetrating  rays  from  the  earth,  207 
Crookes,  Sir  W. 

discovery  of  scintillations,  12 


INDEX 


281 


separation  of  UrX,  161 
Crystallization 

effect  of,  on  activity  of  uranium,  163 
Curie,  Mme. 

separation  of  radium  and  polonium,  7 

period  of  polonium,  1 44 

absorption  of  a  rays  from  polonium,  236 
Curie,  P. 

period  of  radium  emanation,  72 
Curie,  P.  and  Mme. 

separation  of  radium,  7 

discovery  of  excited  activity,  12  and  95 
Curie  and  Danne 

diffusion  of  radium  emanation,  82 

volatility  of  active  deposit  of  radium, 

117 
Curie  and  Dewar 

production  of  helium  by  radium,  182  et 

seq. 
Curie  and  Laborde 

heating  effect  of  radium,  13  and  247 
Current 

through  gases,  24  et  seq. 

variation  of,  with  voltage,  26 

measurement    of,   by    electroscope,  28 
ct  seq. 

measurement  of,  by  electrometer,  32 

DADOURIAN 

presence  of  thorium  emanation  in  soil, 

199 
Danne 

presence  of   radium   without   uranium, 

155 
Danne  and  Curie 

diffusion  of  radium  emanation,  82 
decay  curves  of  active  deposit  of  radium, 

111 
effect  of  temperature  on  active  deposit 

of  radium,  117 
Debierne 

separation  of  actinium,  9 

production   of    helium    from   actinium, 

183 
Deposit,  active,  of  actinium 

analysis  and  properties  of,  166 
Deposit,  active,  of  radium 
rapid  transformations  of,  95 
slow  transformations  of,  122  et  seq. 
Deposit,  active,  of  thorium 

connection  of,  with  emanation,  45  et  seq, 
analysis  of,  48  et  seq. 


Des  Coudres 

magnetic  and   electric   deflection  of  a 

rays,  221 
Deslandres 

spectrum  of  helium  from  radium,  182 
Dewar  and  Curie 

production  of  helium  by  radium,  182 
Diffusion 

of  radium  emanation,  81 

of  actinium  emanation,  165 
Disintegration 

account  of  theory  of, 

list  of  products  of,  169 

helium  a  product  of,  1 79  et  seq. 

of  matter  in  general,  217 

possible  causes  of,  266  et  seq. 
Dissipation  of  charge 

apparatus  for  measurement  of,  208 

effect  of  conditions  on  rate  of,  212 
Dolezalek 

electrometer,  31 
Dorn 

discovery  of  radium  emanation,  70 

EARTH 

radioactivity  of,  196  et  seq. 

internal  heat  of,  213  et  seq. 

amount  of  radium  in  earth,  215 
Ebert 

ionization  apparatus,  209 

number  of  ions  per  c.c.  of  atmosphere,  209 
Ebert  and  Ewers 

emanation  from  the  earth,  200 
Electrolysis 

of  active  deposit  of  thorium,  53 

of  ThX,  65 

of  active  deposit  of  actinium,  167 
Electrometer 

use  of,  31 

Dolezalek,  31 

direct  reading  modification,  33 
Electron 

discovery  of,  2 

variation  of  mass  of,  with  speed,  10 

identity  of  $  particle  with,  10 

number   of  electrons  emitted  from  oi:e 
gram  of  radium,  22 

emission  of  slow  moving  electrons  from 
emanation,  121  and  245 

theory  of  matter,  258  et  seq. 

radiation  from,  261 

models  of  atoms,  263 


282 


INDEX 


Electroscopes 

construction  and  use  of,  in  radioactive 

measurement,  28 
Elster  and  Geitel 

discovery  of  scintillations,  1 2 

radioactivity  of  earth  and  atmosphere, 
16  and  197  et  seq. 

emanation  in  earth  and  atmosphere,  200 

radioactivity  of  soils,  202  <>t  seq. 

effect   of   meteorological  conditions  oil 

radioactivity  of  atmosphere,  203 
Emanation 

discovery  and  properties  of,  12 

emission  of,  from  elements,  22 
Emanation  of  actinium 

discovery  arid  properties  of,  9  and  164 

experimental  illustration  of,  165 
Emanation  of  radium 

discovery  and  properties  of,  70  et  seq. 

occlusion  of,  71 

period  of  decay  of,  71 

rate  of  production  of,  73  et  seq. 

condensation  of,  76  et  seq. 

diffusion  of,  81  et  seq. 

physical  and  chemical  properties  of,  84 

volume  of,  85 

spectrum  of,  89 

heat  emission  of,  90  and  247 

summary  of  properties  of,  93 

production  of  helium  from,  179  et  seq. 

presence  of,  in  earth  and  atmosphere, 
196  et  seq. 

amount  of,  in  atmosphere,  204  et  seq. 

heating  of  o  rays  from,  ,247  et  seq. 

gases  produced  by,  253 
Emanation  of  thorium 

properties  of,  38  et  seq. 

conditions  of  escape  of,  39 

chemical  nature  of,  40 

rate  of  transformation  of,  41 

condensation  of,   40,  80 

connection  of,  with  active  deposit,  46 
et  seq. 

production  of,  by  ThX,  63 
Emanmm 

(See  Actinium) 
Eve 

infection  of  laboratories  by  radium,  135 

amount  of  radium  emanation  in  atmos- 
phere, 204 

collecting  distance  of  charged  wire  in 
atmosphere,  207 


ionization  produced  by  the  radium  ema- 
nation, 211 
Ewers  and  Ebert 

emanation  from  the  soil,  200 
Excited  radioactivity 

(See  Active  deposit) 

7  RAYS 

discovery  of,  11 

nature  of,  20 

connection  of,  witli  (3  rays,  25 

measurement  of,  29 


evolved  by  radium,  253 

Gates 

effect  of  temperature  on  active  deposit 
of  thorium,  54,  117 

Geitel 
natural  ionization  of  air,  196 

Geitel  and  Elster 
discovery  of  scintillations,  12 
radioactivity  of  earth  and  atmosphere, 

16  and  197  et  seq. 

emanation  in  earth  and  atmosphere,  200 
radioactivity  of  soils,  202  et  seq. 
effect  of  meteorological  conditions  on 
radioactivity  of  atmosphere,  203 

Giesel 

separation  of  radium,  9 
separation  of  emanium,  9,  165 
deflection  of  &  rays,  9 
emanation  from  actinium,  165 
separation  of  actinium  X,  167 
phosphorescent  light  of  emanium,  274 

Gockel  and  Zolss 

relation  between  potential  gradient  and 
dissipation  in  atmosphere,  212 

Godlewski 

occlusion  of  radium  emanation,  76 
diffusion  of  uranium  X,  163 
radiations  Irom  actinium,  166 
separation  oi  actinium  X,  167 

Goldstein 

canal  rays,  18 

Gonders,  Hofmann,  and  Wolfl 
experiments  with  radiolead,  145 

HAHN 

separation  of  radiothorium,  68 
separation  of  radioactinium,  168 
magnetic    deflection    of    a    rays    from 
radiothorium,  229 


INDEX 


283 


Heat 

rate  of  emission  of,  from  radium,  13 
rate  of  emission  of,  from  emanation,  90 
emitted  by  products  of  radium,  247  et 
seq. 

Heaviside 
electrical  mass,  10 

Helium 

history  of  discovery  of,  179 
production  of,  by  radium,  181 
connection  of,  with  a  particle,  183 
rate  of  production  of,  by  radium,  186 

Hillebrande 

gases  liberated  from  radioactive  miner- 
als, 179 

analysis    of   radioactive   minerals,    190 
et  seq. 

Himstedt 

emanation  from  thermal  springs,  201 

Hofmann,  Gonders,  and  Wolfl 
experiments  with  radiolead,  145 

Huggins,  Sir  W.,  and  Lady 

spectrum    of    phosphorescent    light   of 
radium,  274 

IONIZATION 

production  of,  by  X  rays,  3 

production     of,    by     radioactive     sub- 
stances, 5 

influence  of  theory  of,  on  development 
of  radioactivity,  18 

methods  of  measurement  of,  27  et  seq. 

produced  by  a  ray,  235  et  seq.  and  271 

variation  of,  witli  distance  for  o  particle, 
237 

curves  for  radium,  239  et  seq. 

rate  of  production  of,  in  various  gases, 
244 

of  water  by  radium  rays,  253 
Ions 

condensation  of  water  upon,  3 

charge  carried  by,  3 

recombination  of,  25 

delicacy   of  electrical    method    for   de- 
tection of,  35 

number  present  per  c.c.  in  atmosphere, 
208 

rate  of  production  of,  in  air,  209 

presence  of  slow  moving  in  air,  210 

rate  of,  production  of  by  radium,  253 

absorption  of  energy  in  production  of, 
271 


KAUFMANN 

variation  of  mass  of  electron  with  speed, 

10 
Kelvin 

internal  heat  of  earth,  213 

models  of  atoms,  263 
Kleeman  and  Bragg 

absorption  of  a  rays,  236  et  seq. 

ionization  curves  for  the  a  rays  from 

radium,  240  et  seq. 
Kunzite. 

phosphorescence  of,  78 

LABORDE  and  Curie 

heat  emission  of  radium,  13,  247 
Langevin 

presence  of  slow  moving  ions  in  atmos- 
phere, 210 
Larmor 

electronic  theory,  4 

radiation  from  moving  electron,  261 
Lead,  radioactive 

(See  Radiolead) 
Lenard 

cathode  rays,  1 
Lerch,  von 

electrolysis  of  active  deposit  of  thorium,  53 

electrolysis  of  ThX,  65 
Life 

of  radium,  148  et  seq. 

of  radioelements,  175 
Lockyer,  Sir  Norman 

discovery  of  helium  in  the  sun,  179 
Lorentz 

electronic  theory,  4 
Luminosity 

spectrum  of  phosphorescent  light  from 
radium  and  emanium,  274 

MACIIE  and  von  Schweidler 

velocity  of  ions  in  the  atmosphere,  210 
Mackenzie 

magnetic   and   electric   deflection   of   a 
rays,  221 

dispersion  of  a  rays  from  radium  in  a 

magnetic  field,  232 
Makower 

diffusion  of  the  radium  emanation,  82 
Marckwald 

preparation  of  a  radium  amalgam,  8 

separation  of  radiotellurium,  9,  140 

period  of  radiotellurium,  139 


284 


INDEX 


Mass 

of  electron,  2,  10 

apparent  mass  of  a  particle,  184 
Materials 

radioactivity  of  ordinary,  217 
McClelland 

secondary  radiation  due  to  B  and  y  rays, 

246 
McClung 

range  of  ionization  of  rays  from  radium 

C,  239 
McCoy 

activity  of  uranium  minerals,  152 
McLennan 

radioactivity  of  snow,  200 

penetrating  radiation  from  earth,  217 
McLennan  and  Burton 

emanation  in  petroleum,  202 
Methods  of  measurement 

in  radioactivity,  23  et  seq. 

comparison  of  photographic  and  electri- 
cal, 24 

description  of  electrical,  24  et  seq. 
Meyer  and  Schweidler 

period  of  radiotellurium,  139 

radioactive   transformations    of    radio- 
lead,  146 

effect  of  crystallization  of  uranium  on 

its  activity,  163 
Minerals,  radioactive 

final  products  present  in,  192 

age  of,  187 

amount  of  lead  and  helium  present  in, 

191 
Moore  and  Schlundt 

separation  of  ThX,  65 

OCCLUSION 

of   emanation   in  radium  and  thorium 

compounds,  74 
Oxygen 

production  of,  in  radium  solutions,  253 

PEGRAM 

electrolysis    of    active    products    from 
thorium,  53 

heating  effect  of  thorium,  252 
Phosphorescence 

produced  by  radium,  23 

produced  by  radium  emanation  in  sub- 
stances, 78 

connection  of,  with  ionizatiou,  226 


spectrum    of    phosphorescent    light  of 

radium  compounds,  274 
Photographic 

method  of  measurement,  24 

connection  of  photographic  action  with 

ionization,  226 
Pohl  and  Walter 

luminosity  of  gases  exposed  to  a  rays> 

274 
Polonium 

separation  of,  7 

period  of,  143 

identity    of,  with    radiotellurium    and 
radium  F,  143 

slow  moving  electrons  from,  245 
Products,  radioactive 

from  thorium,  67 

from  radium,  130 

from  uranium,  163 

from  actinium,  168 

amount  of,  in  radium,  142 

properties  of,  173 

chemistry  of,  173 

RADIATIONS 

from  active  bodies,  19  et  seq. 

from   different   active   products,  1 70  et 
seq. 

connection  of,  with  heat  emission,  247 
Radioactininm 

separation  and  period  of,  168 
Radiolead 

discovery  of,  9 

radioactive  analysis  of,  144 

connection  of,  with  radium  D,  145 
Radiotellurium 

discovery  of,  9 

amount  of,  in  radium  minerals,  141 

period  of,  139 

identity  with  radium  F  and  polonium,. 

139 
Radiothorium 

discovery  and  properties  of,  68 
Radium 

discovery  of,  7  et  seq. 

properties  of,  8  et  seq. 

amount  of,  in  uranium  minerals,  8 

radiations  from,  9  and  20 

emanation  from,  12 

properties  of  emanation  from,  70  et  seq* 

recovery  of  activity  of,  73 

occlusion  of  emanation  in,  74 


INDEX 


285 


condensation  of  emanation  of,  76  et  seq. 

diffusion  of  emanation  of,  81  et  s^q. 

volume  of  emanation  of,  85  et  seq. 

spectrum  of  emanation  of,  89  et  seq. 

heat  emission  of  emanation  of,  90  et  seq. 

excited  activity  produced  by,  95  et  seq. 

decay  curves  of  excited  activity  of,  99 
et  seq. 

theory  of  successive  changes  in,  104  et 
seq. 

analysis  of  active  deposit  of,  104  et  srq. 

effect  of  temperature  on  active  deposit 
of,  116  et  seq. 

slow  transformation  products  of,  122  et 
seq. 

properties  of  radium  D,  E,  and  F,  122 
et  seq. 

variation  of  activity  of,  over  long  inter- 
val, 137  et  seq. 

identity  of  radium  F  with  radiotellurium, 
138  et  seq. 

identity  of  radium  F  with  polonium,  143 
et  seq. 

connection  of,  with  radiolead,  144 

life  of,  148  et  seq. 

origin  of,  151  et  seq. 

amount  of,  in  minerals,  155  et  seq. 

growth  of,  in  uranium,  157  et  seq. 

connection  of,  with  uranium  and  actin- 
ium, 176  et  seq. 

production  of  helium  from,  179  et  seq. 

age  of  radium  minerals,  187  et  seq. 

chemical  constitution  of,  193  et  seq. 

presence  of,  in  earth  and  atmosphere, 
196  et  seq. 

amount  of,  in  atmosphere,  204  et  seq. 

heating  of  earth  by,  213  et  seq. 

properties  of  a  rays  from,  219  et  seq. 

ionization  produced  by  a  rays  from,  235 
et  seq. 

heating  of  a  rays  from,  247  et  seq. 

gases  evolved  by,  253  et  seq. 

processes  occurring  in,  256  et  seq. 
Radium  A 

nomenclature,  95 

analysis  of,  101  et  seq 

effect  of  presence  of,  in  decay  curves, 
112  et  seq. 

connection  of,  with  radium  B,  1 14  et  seq. 
Radium  B 

analysis  of,  101  et  seq. 

effect  of  temperature  on,  1 1 6  et  seq. 


true  period  of,  119 

£  rays  emitted  from  (footnote,  115) 
Radium  C 

analysis  of,  101  et  seq. 

emission  of  j8  and  y  rays  by,  102  et  seq. 

effect  of  temperature  on,  116  et  seq. 

true  period  of,  119 

violent  disintegration  of,  169  et  seq. 

properties  of  a  rays  from,  222  et  seq. 
Radium  D 

nomenclature,  122 

analysis  of,  122  et  seq. 

effect  of  temperature  on,  126 

properties  of,  129 

period  of,  131  et  seq. 

presence  of,  in  old  radium,  136 

connection  of,  with  radiolead,  144  et  seq. 
Radium  E 

analysis  of,  122  et  seq. 

period  of,  125 

effect  of  temperature  on,  126 

connection  of,  with  radium  F,  128 
Radium  F 

analysis  of,  122  et  seq. 

rise  of  activity  due  to,  124 

effect  of  temperature  on,  126 

separation  of,  by  bismuth,  127 

period  of,  127 

variation  of  activity  due  to,  133 

presence  of,  in  radium,  136 

identity  of,  with  radiotellurium,  138  et 
seq. 

identity  of,  with  polonium,  142  et.seq. 

amount  of,  in  uranium,  140  et  seq. 
Rain 

radioactivity  of,  200 
Ramsay,  Sir  W. 

discovery  of  helium,  179 

atomic  weight  of  helium,  180 

amount  of  helium  in  atmosphere,  180 
Ramsay  and  Collie 

spectrum  of  radium  emanation,  89 
Ramsay  and  Soddy 

volume  of  emanation  from  radium,  87 

production  of  helium  by  radium,  181 

rate  of  production  of  helium  by  radium, 
186 

evolution  of  gases  from  radium,  253 
Rayless  transformation 

character  of,  171 
Rontgen 

discovery  of  X-rays,  1 


286 


INDEX 


Runge  and  Bodlander 

evolution  of  gases  from  radium,  253 

SACK OR 

period  of  radium  emanation,  72 
Schlundt  and  R.  B.  Moore 

separation  of  ThX,  65 
Schmidt 

discovery  of  radioactivity  of  thorium,  7 
Schmidt,  W.  C. 

emission  of  ft  rays  from  radium  B,  115 
Schuster 

number  of  ions  per  c.c.  of  atmosphere, 

209 

Schweidler  and  Meyer 
period  of  radiotellurium,  139 
radioactive  analysis  of  radiolead,  146 
effect  of  crystallization  of  uranium  on 

its  activity,  1 63 
Schweidler  and  von  Mache 

velocity  of  ions  in  the  atmosphere,  210 
Scintillations 

discovery  of,  in  zinc  sulphide  screen, 

12 
use  of,  to  determine  range  of  a  particles, 

226 

connection  of,  with  ionization,  226 
Searle 

electrical  mass,  10 
Secondary  rays 
from  radioactive  bodies,  20 
emission   of,  in  form   of  slow  moving 

electrons,  245 
Simpson 

effect  of  meteorological  conditions  on 
amount  of  emanation  in  atmosphere, 
212 

Slater,  Miss 
effect  of  temperature  on  active  deposit 

of  thorium,  53 
emission  of  slow  moving  electrons  from 

emanations,  121 
Snow 

radioactivity  of,  200 
Soddy 

production  of  radium  by  uranium,  158 
Soddy  and  Ramsay 

volume  of  emanation,  87 
production  of  helium  by  radium,  181 
rate  of  production  of  helium  by  radium, 

186 
evolution  of  gases  by  radium,  253 


Soddy  and  Rutherford 
development   of   disintegration   theory, 

13  et  seq. 
emanating    power    of    thorium     com 

pounds,  39 
chemical  nature  of  thorium  emanation, 

40 

separation  of  ThX,  56 
period  of  radium  emanation,  71 
condensation  of  emanations,  76  et  seq, 
physical    and    chemical    properties    of 

emanations,  84 

origin  of  helium  in  minerals,  181 
Spectrum 

of  radium  emanation,  89 

of    phosphorescent    light    of    radium 

bromide,  274 
Strutt 

amount  of  radium  in  minerals,  152 
radium  emanation  in  spring  water,  201 
connection  of  thorium  and  uranium,  177 
radioactivity  of  ordinary  matter,  211 

TEMPERATURE 
effect  of,  on  active  deposit  of  thorium, 

54 
effect  of,  on  active  deposit  of  radium, 

116  and  126 
effect  of,  on  active  deposit  of  actinium, 

166 
Thomson,  J.  J. 

cathode  rays  and  electrons,  2 

charge  carried  by  an  ion,  3 

electrical  mass,  10 

emission  of  slow  moving  electrons  with 

a  particles,  120 

radioactivity  of  well  water,  201 
charge  carried  by  the  a  rays,  245 
electronic  model  of  atom,  264 
radiation  of  energy  from  the  atom,  265 

and  267 
Thorium 

discovery  of  radioactivity  of,  6 
radiations  from,  32 
emanation  from,  38  et  seq. 
emanating  power  of  compounds  of,  39 
excited  radioactivity  from,  45  et  seq. 
transformation  products  of,  48  et  seq. 
separation  of  ThX  from,  56  et  seq. 
separation  of  radiothorium  from,  68 
possible   connection  of,  with   uranium., 
176  et  seq. 


INDEX 


287 


Thorium  A 

period  ami  properties  of,  48  et  seq. 

electrolysis  of,  53 

effect  of  temperature  on,  54 
Thorium  B 

period  and  properties  of,  48  et  seq. 

electrolysis  of,  53 

effect  of  temperature  on,  54 

complexity  of,  242 
Thorium  X 

discovery  of,  13,  56  et  seq. 

separation  of,  56 

decay  and  recovery  curves  of,  57  et  seq. 

emanation  from,  62 

irregularities  in  decay  of,  63 

methods  of  separation  of,  65 

table  of  changes  of,  67 
Townsend 

charge  on  an  ion,  3 
Transformations 

general  theory  of,  14 

connection  of  law  of  decay  of  activity 
with,  42 

of  thorium,  46  et  seq. 

mathematical  theory  of,  50  et  seq. 

of  radium,  94  et  seq,  122  et  seq. 

of  uranium,  161 

of  actinium,  164  et  seq. 

connection  between,  of  radioelements, 
168,  176 

ray  less,  171 

properties  of  products  of,  173 

position  of  helium  as  product  of,  179  et 
seq. 

causes  of  atomic,  267 

processes  of,  269 

of  inactive  matter,  275 

URANIUM 

discovery  of  radioactivity  of,  5 
amount  of  radium  in  uranium  minerals, 

8  and  155 

connection  of,  with  radium,  151 
growth  of  radium  in,  157 


transformation  products  of,  161 

separation  of  UrX  161 

effect  of  crystallization  on  radioactivity 

of,  163 
Uranium  X 

separation  of,  161 
radiations  from,  161 

effect  of  crystallization  on  distribution 
of,  161 

VELOCITY 

of  a  particle  from  radium  C,  229 
retardation     of,    in     passing    through 

matter,  224 
connection  of,  with  range  of  ionization, 

225 
Villard 

discovery  of  7  rays,  1 1 

WALTER  and  Pohl 

luminosity  of  gases  exposed  to  o  rays,  274 
Weichert 

nature  of  X-rays,  2 
Wheeler  and  Burnstead 

period  of  radium  emanation,  72 

diffusion  of  radium  emanation,  82 

radium  emanation  in  atmosphere,  199 
Whetham 

production  of  radium  from  uranium,  158 
Wien 

canal  rays,  18 
Wilson,  C.T.  R. 

ionic  nuclei,  3 

electroscope,  29 

natural  iouization  of  air,  197 

radioactivity  of  rain  and  snow,  200 
Wolfl,  Hofmann,  and  Gonders 

experiments  in  radiolead,  145 
Wood 

radioactivity  of  ordinary  matter,  217 

ZOLSS  and  Gockel 

relation  between  potential  gradient  and 
rate  of  dissipation  in  atmosphere,  212 


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